I'm riding a Surly Cross-check at the moment, in a 56cm. I'm 6'1", so I would normally ride a 58cm road frame, but I found that a lot of CC bikes fit differently and require you go a size smaller than your road bike. I even thought the 55cm Bianchi Axis and Jake the Snake bikes I test rode fit me better than did their 58cm counterparts.
I also think bikes that are a little small handle better offroad--but I'd best had not ought start that argument.
Having ridden the smaller Surly for a couple months now, I'm glad I went that way.
$.02
-M