I recently made the switch from Metro to ST. Whether its a good switch really depends on you. If you can live with the data cap and either ATT or T-Mobile have good coverage in your area then I don't see why not.
If T-Mobile has good coverage in your area then when you're selecting a sim card to order make sure you pick the T-mobile card so you can run off of T-Mobile's towers and have their higher data cap.
It seems the phone will be sold unlocked when it is finally sold in the US so you won't have to worry about doing any unlocking. You will have to enter in APN settings, but don't worry ST gives you all that info and a little guide and its really easy and quick to do.
As for the GPS that really depends on the phone itself, so you'd have to look up any reviews for that. Though since this is suppose to be a top quality phone I suspect the GPS will be pretty snappy.
The talk about no-streaming allowed is left over TOS that ST won't remove because it covers them legally very nicely when they decide to kick someone off of ST for using too much data.
Personally I use ATT's network which I know means a cap of about 2GB a month in data and I've streamed youtube and netflix with no issues. Not that I have done very much streaming or anything though.
As far as I understand ATT has a cap of around 2GB and T-Mobile is around 4-5GB.
I really made the switch because I'm going to be spending several months where Metro only has essentially roaming coverage. Not to mention I wanted the S3, but plan to switch to T-Mobile's $30 plan when I move permanently so a GSM phone was the way to go.