German iPhone 5 in the US

Yes you can use it as long as it's not locked to your German provider. You would get pretty good speeds on the HSPA+ network of T-mobile here as long as you are in a major area that has been refarmed. It would show as 4G on your signal indicator. I think speeds in the 6-20Mbps area or higher are not uncommon. I think some people have reported even higher speeds.

 

it looks like both GSM versions of the iphone 5 have HSPA+ data on all GSM frequencies (850,900,1800,1900 mhz). so it looks like no matter which carrier you choose here, you should be able to get the faster HSPA+ speeds. however, as you already stated, you likely wont get LTE.

 

Yea, even though HSPA+ is technically 3G, both ATT and T-mobile will show this as 4G in the status bar. (They show LTE when you have an LTE connection). T-mobile's HSPA+ speeds rival some LTE connections.

You should get this if you are staying in most major cities....especially any of the cities where they have turned on LTE, because they moved the HSPA+ to the 1900MHz GSM band to make room for LTE on AWS (1700MHz).