I was/am pretty devoted to the UK series, and didn't get around to the US remake until the last few months. I've been powering through the DVDs. I just skipped the first season, as I understood it was a straight, line-for-line remake of the UK episodes.
Only the pilot was a straight remake. It didn't really come into its own until season 2 though, so you didn't miss much.
The only time I can stand Jim is when he humiliates himself, like that episode where he got everyone locked in overnight. Jim and Pam are pretty awful as a snarky power couple, but Tim and Dawn probably would have become just as unbearable had that storyline been stretched out to fill American-length TV seasons. As for the actors, Fischer was terrible early on but improved, while Krasinski has never been particularly authentic as an office schlub, annoying floppy haircut aside.
I like pretty much all the supporting characters, especially Angela. Creed is great but obviously limited so they've managed to use him perfectly. Also, I like the way Dunder-Mifflin is shown to be fraught with incompetence at all levels rather than just the Scranton branch.