They are great, not just at streaming from an iPhone/iPad but also to stream from your iTunes media library from a Mac elsewhere on the same network too. 2nd gen are very desireable, and fetch over £200 due to the fact that there is the jailbreak available as Mandy pointed out.
Once jailbroken they can be setup to receive all sorts of content, including Sky and also from a Plex media server.
No jailbreak in site for the 3rd gen, although i am hopeful.
Even if you just want it for Netflix etc, the price point is unbeatable really. Get one from eBay for £79 ish.