There's already and active thread on going clipless as I'm sure there've been other recent ones. Just FYI.
I recently moved to a full SPD setup and I found that it's changed the way I pedal dramatically. Not only does it make climbing and sprinting better but it also makes the flats feel better. Whether hills or flats, I always feel like I'm putting significantly more power down without using much more energy than I normally might have.
As for fast exits, you'll want to learn to clip out fast. Clipping out isn't hard. Clipping back in quickly takes more practice but both come naturally with time and practice, much like anything else in life.
If you're going for more speed and are interested in putting more power down more efficiently then I say go for it. They seem meant to be on a ride bike or MTB. Anything else would just be funny (IE, like a trike or a cruiser).