My friend was so excited for this that he decided to order the Japanese version. PS3's aren't region-locked! We serve a living God!
KH1.5's oldness (gameplay-wise) is palpable, but what can you expect? Still, it's the workhorse of the ReMiX and it is worth the buy if you want to relive the rush of emotions that is the Night of Fate, or go through Hollow Bastion with a wooden sword again (in HD.)
My friend also had a save file that let me experience the ultimate HD ass-whooping from the secret boss.
I've never cared for 358/2 days and I don't think I should, so I didn't bother with it. I'm sure the cut scenes are enjoyable though, and I can see why Squenix didn't leave the gameplay in.
I wasn't a fan of CoM on the GBA either but it is nice to see this game in 3D!