I just got back from a ride where I was running flat pedals on a bike on which I normally use clip pedals. While the ride wasn’t a wash, it certainly was an eye opener for me, especially since I haven’t used flats on any bike for a couple of months.
The first and most obvious difference between the two types of pedal is efficiency. For pedalling uphill AND downhill, clip-based pedals are far more efficient than flats because you gain the power from the downstroke (push) AND upstroke (pull) with clips; using flats, you only receive the benefit of the downstroke. Also, with clips your feet are in constant contact with your bike; with my flats I found that my feet kept slipping off the pedals on climbs and on parts of the descent, regardless of how much I focused on keeping things together. I’m sure that my problems were a function of the particular pedals I was running and my not being used to them but they really weren’t all that fun.
That being said, flats do have positive characteristics. They allow you to move your foot around on the pedal without unclipping. For some people, this trumps the efficiency of clip-in pedals. Flat pedals also allow you to wear shoes that are comfortable to walk in. Regardless of what purists might say, there are sections of trail out there that just aren’t possible to ride and trying to navigate those steep and rocky slopes in burly-looking tap dancing shoes is no small feat.
Many riders who use flats are concerned that their bodies won’t disengage from their bikes should they crash while clipped in. In the past, when certain pedals (ahem Shimano ahem) would cement your feet in place that was a legitimate concern but now, with mud-shedding pedals like the Time ATAC, the Crank Brothers series, and the higher end Shimano pedals, getting stuck to your bike in a crash just doesn’t happen very often.
That being said, deciding what type of pedal to run on your bike boils down to personal preference. While I would recommend that anyone who rides a cross country oriented bike should invest in a decent set of clip-based pedals I understand why many people who are into freeride, street, and downhill are more comfortable with flats. If you’re thinking about switching over to one type of pedal or another, see if you can convince a friend to lend you a set of their pedals; if you like what you test, consider making the plunge and getting on with it. Just keep in mind that it’s not a contest — riding with clips doesn’t make someone a better rider than someone who doesn’t use them. For most of us, it’s all about finding the gear that gives you the most enjoyment when you’re on your bike.