I'm a fan of flats at this point, primarily for the on the fly micro adjustments. When switching over, I experienced quickly the technical benefits of dropping the heels, and since then I rare get bucked off the pedals. I didn't realize how often I'd be driving pedals with toes pointed downwards, but flats fixed that right quick.
One huge plus for cleats: If you're prone to twisting ankles, they're great for a quick recovery. I used to play a lot of basketball, and when I'd land awkwardly and get a significant sprain, I'd be off the court and the bike for several weeks at a time, hoping to avoid another tweak while oh so slowly healing. In 2003 I was spending ~equal time on trails and courts, and started riding SPDs. Missing the cardio of both, I decided to go for a ride just a few days after a good sprain. Two days later my ankle was strong and stable, and I was back playing basketball and riding good riders within a week. The same situation played out a half dozen more times, with the same recuperation results, and I attribute the quick recovery to the unique combination of getting the good work that pedaling provides without the potential to roll the ankle again (assuming I was careful when starting and stopping, and didn't ride where I might dab on a round rock).