Originally Posted by DMF
Or not. 6mm 'long' Allen keys are about 4" long. 15mm wrenches are about 8" long. If they were put on by a wrench, the pedals will probably be too tight to take off with an Allen key.
Use a 15mm wrench. An actual pedal wrench is the best solution, but they - especially the big Park Tools wrenches - can be pricey.
Do what DannoXYZ says and get a set of metric wrenches.
Or yes. It's worked for me. And you can fit a small piece of pipe over the wrench to increase leverage. Could be the bikes I've successfully used allen keys on were rarely used, and the pedals not torqued very tightly. Why would there be allen recesses anyway?
The advantage to allen wrenches is access. With some pedals, it's difficult to get a conventional wrench between the crank arm and the pedal platform, or the flats are too narrow for conventional wrenches. (I know, that's an advantage to cycling specific pedal wrenches.) However, my other point is that some very cheap pedals often have odd sized flats. I've encountered flats on cheap pedals that are smaller than 15 mm and larger than 14mm, and which don't fit standard wrenches either.