Dunno. I've never been interested enough to pick up one of his books. If you do, you can let me know what I'm missing.
I read The Connections and liked it a lot. He's a good writer that writes "Realistic Novels" and I found it very entertaining. If memory serves, my only quibble was that the book's morals were very well defined: any "transgressions" were paid for at the end of the book, rather than allowing for some ambiguity (one of the great things about Little Children, another great realistic novel, was the multifaceted way Perotta presented a pretty awful character). I recommend it, but I can't say I know any of his other books, although Freedom is on my list of must reads; the only reason haven't picked it up yet because I was waiting for the hype to die down so I could pick up a copy for a song at a used bookstore.
FWIW, here's more on Franzen from Charlie Alderman, speaking about my point above and making a connection between Franzen and DFW.
read the corrections. the book started off great.... it was tragic in the sense of taking a perfect white dress and dunking it in a mud puddle. but the rest of the book was simply taking that muddied white dress and continuing to dunk it in the mud puddle with different angles, speeds and depths. i get it... dirty dress. some times stories are better if they leave a little more to the imagination.