Mine has very worked well. But is not perfect.
Pros include not having expensive equipment in the car to get nicked.
I have jumped in and out of numerous cars - including my mate's and hire cars - and been able to tune in the device to a free FM frequency, and then tune the stereo to match.
I find a higher frequency is better as one car had electrical noise that was also being picked up.
The remote control is useful, especially if you memorise where your favourite albums start. My remote is attached to the dash with velcro.
Cons include not being able to count beyond 255 tracks. It will play the tracks 256 and above, but when you stop the car it can't remember where it was last playing once it has got past 255. (That is 20 odd albums though)
And the tiny display is unreadable when driving. Especially when ciggie lighter is low on the dash or the sun catches it.
This means it is best used with a handful of SD Cards stored in a wallet. I use 1GB and 2GB cards as they are again cheap. Putting stickers on my SD Cards to identify them. No car journey has been longer that a card's contents yet
Easy to swap while driving
Winamp is great for setting the cards up. Especially as my music collection is stored as FLAC on the server. Winamp's ability to convert on the fly is perfect for filling the cards up with MP3s.
Another oddity of the cheap device is it will play tracks in the order you copy them onto the device. So care needs to be taken at transfer time.
All of those cons can be bypassed if you have an MP3 player you can attach to the side of the device through that headphone lead. Or if your mate jumps in the car with his iPod and wants to play some tracks.
Summary? Can't beat it for the price.