Ok, thoughts on the first issue.
With books that begin like this, it's always tempting to say that everything is there from the first issue; that the book arrives fully-formed with recognisable DNA from the start. And that was my feeling after re-reading issue #1 for the first time in years the other day.
But after thinking about it a while, I'm not sure it's true.
Once you've read the entire series, you can look back on the first issue and see seeds there for lots of import stuff that was to come later. But that's all they are at this point: seeds. Hints towards bigger things to come that are enough to be intriguing, but without enough detail to really give you a handle on much of anything yet.
What there is though, is some very efficient writing from Ellis that sets up the book's concept and main characters in a very short space of time, and then proceeds to tell a complete(ish) story with them.
(Yes, as David points out, that was done all the time in the Silver Age, but in the more modern and less compressed style that Planetary adopts, it's impressive.)
One of Ellis' skills is to imply a lot with a little, and he does that countless times here. The three Planetary agents feel well-defined even though we don't have much to go on in terms of detail: Snow's iciness and caginess and Jakita's attitude and power in particular.
Also, the pulp heroes all carry the sense of having long, fully-formed histories even though they're mostly just a collection of archetypes fleshed out only through fairly sparing dialogue. Even the multiversal superheroes feel pleasingly familiar, with a smart deployment of clear analogues for a very famous team.
It's all shorthand, but it works, and assumes a certain familiarity with pulp/comics history on the part of the reader to really bring it all to life.
While this first issue doesn't really convey the full scale and scope that Planetary would go on to have, it's suggestive of enough to make you intrigued to come back for more next month - which is maybe the most important thing for a first issue, really.
Now, having talked a lot about Ellis, I think it's worth considering Cassaday's contribution. I think the designs for this series are really strong, and the three leads introduced here feel as though they're defined at least as much through the visuals as through the dialogue - both in terms of their costumes and general look, and in terms of subtler things like body language, posture, facial expressions and so on.
Also, I love the touches of realism that Cassaday brings to the page. Little things like the small impact-craters that Jakita makes when she jumps to the ground from the helicopter are a good example (and while they were called for by Ellis - as the script to issue #1 that's printed in the back of the omnibus shows - it's Cassaday who makes them work).
Another thing revealed by the script is that Ellis had very specific ideas for how the pulp heroes should look, but he then gave Cassaday carte blanche to come up with his own designs for the JLA analogues (although Ellis dictated which characters they should be based on). I like that Cassaday went so crazy and offbeat with them. They're quite futuristic I think - it contrasts nicely against the old-fashioned pulp heroes.
Other great visual concepts include big stuff like the Snowflake - where Cassaday actually departs from Ellis' script to a small extent, by saving the explicit reveal of the tiny Earths until a few panels after Ellis calls for them to be seen. I guess he had a strong sense of when that detail needed to be revealed.
There are also loads of great smaller throwaway details - would the Murder Colonels feel anywhere near as weird and wonderful without Cassaday's illustration? And would the waitress of the first few pages be anywhere near as memorable without that wonderful panel of her pissed-off scowl?
While it might not quite provide a fully-formed version of the series that Planetary would eventually become, there's enough in this first issue to indicate that Ellis and Cassaday are working in tandem together very well (had they collaborated before this? I forget), and that there's lots of scope for where the series could go in future issues. It might not be a perfect first issue, but I think it's clearly something fairly special from the off.