The biggest mistake King made with him was drawing out the mystery of his identity before Brainiac recruited him, which wasn't revealed until the Telos series itself, and then not even the first issue. Writers like mysteries like that. Impatient fans don't.
What I liked most about Convergence was that it was a fun little continuity story, drawing on years of unrelated stuff and integrating it, like a reverse Crisis. It led directly to the current Superman status quo, besides, which will probably be its lasting legacy. And it was basically an Earth 2 event, which I also appreciated, even though most fans didn't. They found it, especially the World's End weekly series and Society follow-up, a jumbled mess, but I found the results to be refreshing.