So, production on this 2-parter is pretty convoluted. Episode 1 is the series 3 finale, and the original plan was to do the story that became Year of Hell. Then the decision came down to change the cast a bit, and Year of Hell was pushed back later into series 4.
Another proposed finale was a copy of Voyager crewed by biomimetic lifeforms arriving at Earth, and wreaking havoc before they're found out. Unlike Year of Hell, this story was never developed into a full episode on its own, though the biomimietic lifeforms would be used as the basis for Demon and Course: Oblivion
The idea to use the Borg as a big opponent came from Brannon Braga. Unity was the only planned Borg episode and he felt that in the aftermath of First Contact, it was time to bring them back to TV as well. Braga also came up with the idea of Species 8472.
Fan speculation in the hiatus between series 3 and 4 was that Garret Wang and/or Jennifer Lien would leave the show, especially Wang given Kim's grave injuries at the end of the episode. And... well, nobody really liked Kes.
Apparently it was decided at one point that Wang would be the one to go, but he showed up in People Magazine's 50 most beautiful people list, and apparently the producers decided to keep him around as a result.
As a result of the casting decisions Lien's last episode as a regular is Scorpion Part 1. For Part 2 Jeri Ryan is listed in the main cast, and Lien has a guest credit. Her final episode was The Gift - the episode immediately after Scorpion, and she made a final guest appearance in the Series 6 episode Fury
Upon Lien's departure, Kate Mulgrew spoke very highly of her talents as an actress, and during retrospectives pretty much the entire Series 1-3 cast spoke of how much they missed her and how her departure effected the tight-knit family feel the central cast had developed.
While Mulgrew predicted Lien would become a movie star, her post-Trek career was quite spotty, with a handful of movie and voiceover roles in the late 90s-early 2000s. In more recent years she's only made the news as a result of various run-ins with the law.
Part 1 also sees the first appearance of Janeway's Leonardo DaVinci holodeck program, with John Rhys-Davies as Leonardo. Rhys-Davies is a long-standing veteran of genre TV and movies, most notably as Gimli and Treebeard in the Lord of the Rings movies, Sallah in the Indiana Jones movies, Professor Arturo in Sliders, King John in Robin of Sherwood, as well as many voice over roles.
Scorpion marks a point where Voyager began to shift away from the majority of the show's effects being optical to CGI. Foundation Imaging, best known for producing the CGI for the first three years of Babylon 5, had begun to work on the show at the start of the year, creating a sandworm for Basics Part 2 and a swarm of starships for the episode of the same name.
Most of the CG for this episode was provided by Foundation, under the Voyager's lead effects supervisor Ronald B Moore (no relation to writer Ronald D Moore). In practice Moore handled the practical work, and shooting of live-action elements of composite shots such as when the Species 8472 pilot attacks Harry Kim in the Borg Cube.
The battle between the Borg Cubes and Specis 8472 in the second episode wasn't part of the original script, but a sequence Foundation did on their own initiative as Ron Thornton felt it would be an awesome addition to the episode. He was also very proud of the sequence where the Borg are sucked out into space.
In the other direction, the pile of Borg corpses Kes saw in her visions and was found on the destroyed Borg Cube by Chakotay, Kim and Tuvok was in reality a twelve-inch tall pile of Playmates Borg toys, cut up and the parts glued together. They were photographed against a blue background and composited into shots of the set.
As may be evident, many of the Borg costumes and set pieces were originally made for First Contact. Similarly, the matte painting used for the shots of Janeway talking to the Collective at the end of Part 1 was from First Contact.
As noted, Part 2 was Jeri Ryan's first episode as a series regular. She was cast 6 weeks before the shooting began on the episode, and fittings for her costume began almost immediately. The full Borg prosthesis was very tight and pinched on Ryan's carotid artery, causing her to almost pass out during a long day's shooting.
Ryan's first few days on the set saw her bonding greatly with the cast. She met Garret Wang and Tim Russ in the makeup department during her costume fittings, but met most of the rest on her first day. She fit in well with the cast's sense of humour (though every time she laughed it made her eye prosthetic fall off), and this helped her past a huge dose of anxiety, exacerbated by the fact that she couldn't see where she was going very well, and didn't have a good sense of the bridge set's ins and outs, and as a result spent a lot of time stumbling and tripping over things.
There was a lot of fan trepidation about Ryan's casting, especially after promo photos of Seven's silver catsuit costume were released. Jeri Ryan said in 1999 that she was actually getting hate mail before the series began. Similarly Robert Beltran was a guest at a con a few weeks before Part 2 aired and he urged fans to give Seven a chance and said that they would like her and her addition would be beneficial to the show.
Also speaking of actors talking about conventions, during the hiatus between series 3 and 4 Kate Mulgrew was attending a con where she spoke of how much she enjoyed Part 1, and was excited for Part 2 as she had no idea what was going to happen.
The 2-parter was highly regarded by cast and crew, with pretty much everyone speaking very highly of it. Garret Wang noted that he liked the idea of an enemy even the Borg couldn't defeat, and Jeri Taylor noted that the show's popularity vindicated the big risks it took.
While Taylor thought during her comments that Part 1 was the highest-rated episode of series 3, she was mistaken - Future's End, Part 2 had the same numbers but a higher audience share, and Basics, Part 2 was higher-rated on both fronts. It was the most popular show for the entire month on Sky One when it first aired in the UK, however.
This episode marks the first instance of conflict between Janeway and Chakotay that would cumulate in him considering mutiny against her in the Series 6 finale/Series 7 opener Equinox.
The scene where The Doctor and Kes discuss assimilation gives an explicit explanation of how the Borg assimilated various Enterprise crewmembers in First Contact
The neural probes the Borg forcibly attach to Janeway and Tuvok and Chakotay uses to connect to Seven were first scene in Unity.
The asymmetrical ship used by the rogue Borg in Descent appears on a screen in the Borg Cube as a mine schematic.
And so: opinion!
This is a hard episode to assess for me, because while it's a good episode by Trek standards, not just Voyager ones, it's the point where I fell out of love with Trek in general. I never saw Part 2 when it aired on TV, even though I watched most of series 4, I wasn't paying attention enough to notice the airing dates. Looking at a list of series 5 episodes, I think I watched less than 5 on TV.
So yeah, this is a good two-parter overall. It plays to Voyager's strengths - they're decades away from any assistance, so they need to be careful, and they need to make alliances that are only marginally better than the alternative. While the first point became a point of derision for the show with their magically replenishing shuttles and torpedoes. it works well here, as Voyager has to retreat a few times due to damage. The latter was played with earlier in Series 2 with the episode Alliances, it's much more stark here. The Borg are the greatest foe the Federation has ever seen, a constantly expanding and hegemonising swarm. But Species 8472 is potentially more dangerous. Giving the Borg the nanoprobe weapon means leaving them a power in the Delta Quadrant, with the ability to reach even further thanks to their slipstream network. That hard call is what Voyager was meant to be about, but sadly rarely lived up to the potential. It's part of why Ron Moore (the writer, not the effects guy) quit the show so quickly after moving over post DS9's finale.
What's especially impressive is that while there is technobabble in the episode, it's in service of the story rather than driving it. it goes against the grain of what TNG became towards the end and what Voyager continued excessively (while DS9 didn't so it as much, it still did it and that was part of what drove me away from Trek too).
Ultimately, it's a great indicator of what Voyager should have been. I far prefer it to, say Year of Hell (that fucking reset button!), but because of that it stands starkly against what Voyager usually was. If only the show could have met this standard more often.