Hmm... Well, I couldn't really decide. You see, I think Reaction Commands (as everyone else said) are visually awesome, 'specially (in my opinion) the Cerberus one >w<! They seemed to replace the 'empty' slot from the first game in most cases, which is a big plus for me, because it's a little easier to tap triangle when you wanna open a treasure chest or whatever.
But my biggest issue with them was the fact that they were limited to one button. That, and they were one of the many reasons (along with Drive Forms, Limits, and countless new abilities) that Kingdom Hearts II was so much easier than its predecessors. Birth by Sleep seems to be incorporating them, but I can't help but wonder if it won't be a repeat of the same mistake... Hopefully not. After all, it seems that they may use the Circle and Square buttons as well, making for a bit more of a challenge. :3
o_o ...Like others said, sometimes they were more of a way to keep from being killed than a way of making the fight that much easier/pretty to look at, especially (I haven't played it-- only watched) in the Final Mix+-exclusive bosses. Though they definitely could use improvement in both timing and variety (they were only tricky once or twice, like in the Xemnas fight), overall they aren't too bad.