I love how everyone is talking about having an 11t cog for top end. I don't know about anyone else but I don't race and typically don't cruise along flats at 30+ mph...I spin out of my 50-13 combo (I run a compact with a 13-29 cassette) at between 33 and 36 mph some place. I've never had an issue keeping up with any group ride because I ran out of gears.

IMO most riders knees would be well served by a compact crank unless they only ride on flatter terrain. My Merckx has a 53/39 and I can ride it just fine but I run the compact on my main bike and prefer it as there is a lot of climbing around these parts. If I was going to go ride someplace flatter I may take the Merckx more often.