I have a 2006 Trek 520. It's set up for fully loaded touring with front & rear racks & fenders, but I use it for my 17 mile round trip commute daily.

I found the stock saddle to be intolerable, so I swapped it for a Brooks. Lots of people say the stock gearing is not low enough for fully loaded touring, but that shouldn't be an issue for commuting...I have the stock gearing and it works for me. Also, the rack is not so good for touring, but it should be fine for commuting. I used it for a while before swapping it for touring purposes.