I changed from SPD's to Flats about a year ago and, although sometimes tempted, haven't gone (and prolly wont go) back to SPD's.
Flats:
Forces better riding technique and body positioning if you are not/ever going to be a riding God. My riding has improved 500% since changing to flats. If I'm not relaxed, keeping my heels back, compressing at the right time etc, my feet come off the pedals. Nowt like that to start learning how to get it right!
Seemingly less injury risk as I mostly bounce well clear of the bike now if I have a big off, or bail sensibly before that happens.
Easier to restart.
Don't need SPD shoes.
Look better (IMO)
Oh and feet do stay on over the rough stuff/jumps/drops once you get the technique.
SPD's:
Far superior for climbing (particularly technical).
Makes speed hopping/riding rough stuff easy because you can pull the bike up
Foot always in the same position on the pedal (probably the only thing now that makes me think about changing back is getting frustrated by not finding the 'sweet spot' on flat pedals and having to keep repositioning my feet until I do. You'd think by now it'll be second nature).