I've played the numbered ones FFI to FFXII, but missed basically all of FFIII and FFXI. So yeah, only the PS and PS2 ones so far. And played/watched some of the others for awhile.
I did enjoy most of them, but as is said above, FFI wasn't great. My only other problem with the ones that I've played is FFX-2 in my opinion it really shouldn't have existed it really did suck that much and made a mockery of the loved jobs system.
But on subject - I don't mind that FFXIII will be on the 360 aswell as PS3, I will however mind if something goes wrong because of that fact and shall haunt all 360 users, starting with my brother. Frankly, it looks like a fantastic game and from what i've read about the storyline it sounds solid enough and im looking forward to the uber battlesystem which make FFXIIs look turnbased heh. So yeah, great disappointment if it sucks.