I don't want to buy a new head unit and I want to listen to my mp3 player so I came across this: http://www.crutchfield.com/S-UU4uztf...50&I=607FMDA25
Has anyone ever used one of these before?

yeah they arent the best.. they cut in and out.. imo its onestly worth it to get a cheaper headunit and use the AUX channel

I found a similar one on amazon for $8, for that much I might as well just pick it up and see how it well it works.

I have Sirius and installed one of these in my Avalanche and it works great ! The sound quality is much better than when I was using the FM Transmitter.

That is an FM modulator. I have six of them in use, all connected to CD players, and they work pretty good. They will cut out if there is an FM radio station broadcasting on the same frequency, however some of them have the capabilities of being tuned to a different frequency if interference is a problem. Charles