I switched to riding SPD from cheap flats.
Cost wise, little more expensive for SPD shoes and pedals, but then the "top" flat pedals aren't cheap at all.
I found that in wet weather I was slipping on the pedals, a stiffer/gripper shoe (I was using Globe Skate Shoes) may have helped, or some better pedals or both. I switched to SPD to improve pedalling efficiency, made a massive difference to my riding and fitness.
I feel that I have to wimp out a bit on some tougher sections where I feel I might fall. I struggled a bit at Gisburn in the wet on the Northshore and some very muddy sections where it was slow going and I wanted to dab out now and again.
As for OMG I HAVE TO PUT SPECIAL SHOES ON, well I would wear special shorts, gloves, helmet anyway, so some shoes isn't much else, and besides, I wouldn't want my normal shoes getting all cacked up. Just wash down my SPD shoes after a ride with a sponge and they come up great every time.