I bought a cheap chinese unit initially on ebay. the sat nav worked great but after that it was utter utter rubbish. the worst thing being it took about 3 mins each time I selected the ipod mode and then had no memory of where it left off if I switched to another function or turned it off. Luckily for me it had a fault with the screen so I got a refund. The seller was very keen that I did not put negitive feedback on ebay. When I got my refund I gave honest feedback. That was a lucky escape. the moral of my story is that I wont ever buy something that looks too good to be true again.
I since bought a JVC AVX840 on the web, it works quite well but does look a little square in the dash (due to removable faceplate). I dont have the sat nav option on mine. If I lived in the US I could download an satnav app and get my iphone to display on the screen ....... but not this side of the pond yet.
It works well with full ipod control, albeit a little slow. Screen is very clear for watching dvds, not that I use it much for that.
The other main reason for installing was for a reverse camera setup, this works very well. Overall I'm quite happy with it.