Interesting idea, bad implementation. Immediate actions are pretty antithetical to a turn based combat system. They have uses in things like AoOs, but you need to either limit use as much as possible (use them only in explicity defined situations, like AoOs) or design the entire system around the concept (AoOs being one mechanism and immediate spells being another just doesn't fly). Because of this, I think they pretty much suck in 3.x, but think they might work in 4e.
I do have some problems with the base concept of them as well. To me, the whole point of a dice-based RPG is to play make believe, and let the dice settle the arguements of "I shot you first!" "No, I dodged it!" Immediate actions basically bring that arguement right back into the game. If two people try to use immediate actions at once, who gets to go first? How soon after someone starts doing something do you have to declare the immediate action to interrupt it (and what if they roll dice before you blurt out to interrupt)? We need much more refined mechanics to make these problems disappear.
