I loved Kingdom Hearts 2. Though the bit at the beginning with Roxas went on a little too long for my liking really.
I found that the fighting was actually easier than the original Kingdom Hearts. I beat Sephiroth on Proud mode, which was something I didn't do in the original.
They've released Kingdom hearts 2 final Mix in Japan, which might be coming over to US and UK, but it'll be a while yet. Hopefully it'll happen as there's much more content in it than US/UK versions. Also includes a remake of Chain of Memories.
As for the length, I found that it wasn't too bad. I mean, I completed most, if not all of the optional stuff, and levelled up quite a bit so that'd take up time, but in my opinion, it was good.