I've been riding my 58cm (largest frame) Trek 520 for 6 weeks now (600 plus miles) on my daily commute. I am 6'3'' tall with a 34" pant inseam. I was reluctant at first to get the 520 because I've been riding a 63cm (largest frame) Trek 1000 since 2006.
I "feel" very comfortable on the 520 and fell in love when I test rode the bike but have 2 concerns. 1.) On a steep, windy path between neighborhoods I have to stand up the last 30 yards and have bumped the bar end shifters with my knees a couple of times if I lean forward too far. 2.) I have felt a front end wobble start if I lift one hand off the bar to look over my shoulder. (like there is too much weight over the front of the bike). The head set is not loose. I had the LBS mechanic check it on my first tune up visit.
I know this is a difficult topic to hash over on a forum without seeing me on the bike but I thought I'd at least throw it out there.