Ever use an unlocked GSM phone on another carrier?

Just wondering how well it would work? I'm just picking my nose and looking at ebay, and notice there's a lot of unlocked phones that sell for much cheaper than what I can buy for my tmobile network. Any experiences?

 

a friend of mine has an unlocked iphone (3g) and he uses tmobile with no problems

 

5 or so years ago I helped a friend set up a SLVR on at&t...had to call and get certain settings for internet...wasn't too bad.

 

I'm using an unlocked T-Mobile G1 overseas and it's awesome. No trouble at all. Just put the SIM card in and its working wonderfully!

 

Unlocked phones are great if you travel a lot. Just go to the local phone company in the country your visiting, insert their pay-as-you go sim card and voila!!!