Personally I think this is still my favourite of the X-Men films - just a shame it botches up the timeline a bit in retrospect! At the time I'd assumed it was supposed to be a reboot rather than a prequel, and am still convinced that this was supposed the case - hence things like the cameos of '60s era Cyclops and Storm in the Cerebro sequence. However, with Days of Future Past presenting First Class as a prequel and folding it into the same timeline as X-Men 1-3 it does mean there are a few continuity glitches in retrospect. That's probably more of a fault of Days of Future Past than First Class itself, though!
It's definitely got my favourite soundtrack of all the X-Men films, too - some wonderful themes, especially the training montage theme and Magneto's theme tune. Although I enjoyed Days of Future Past, I do wish its composer had teamed up with the composer from First Class - imagine that film with the future-set sequences scored in the style of X-Men 1-3 and the past scored in the style of First Class! Could have been a little jarring, but done right it would have been absolutely awesome in my opinion.
I do feel it's a shame that the next film skipped so much time after this one, too - particularly as some of the team established in this film didn't return for the sequel. I'd have loved a Cold-War era movie between them with the whole First Class-era crew vs. Omega Red! Alas, it was not to be. = (