Photojojo put it well:
The results are stunning: a million shades of rust, ghostly-lit interiors, broken sinks, mirrors, typewriters, rotting pillows, a red barber’s chair that looks as new as the walls around it look old. Piles of tiles and wallpaper rot next to the room of broken wheelchairs. Birds nesting in a breaker room. Al Capone’s prison cell, just as he left it.Here is the shot of Capone's cell they mentioned (definitely nicer than some other cells I have seen):
Tom's photos are beautiful and sometimes a little wistful (respectively, below – I want to bring that piano back to my house and love it forever).
This site's definitely worth a visit; there are dozens of buildings to sneak into and hundreds of photos of them. The fact that he only relies on natural light coming through broken windows and cracks in the walls & ceilings really adds to the eerie mood behind these and almost makes you feel like you're there.