Free weights, machines, and smith machines are all a
lot different when it comes to motions. For bodybuilding purposes, a smith machine is an "OK" tool, but if your goal is to get stronger overall, I'd never use one. It's been a while since I've used a smith for benching but I remember about a 50lb discrepency when I tried to make the switch. A very embarrassing 50lbs I might add.
IMO, a forced range of motion can not be a good thing. Your body doesn't press in a
straight line naturally, so why force it to?