Yes I have used it quite a bit and much prefer it to Itunes.
First of all, the files are encoded in MP3, rather than Apple's .aac format, therefore they are almost universally compatible. I have a serious problem with Apple's monopolistic approach to everything where if you buy one thing from them, everything else you want to use it with has to be Apple too. (i.e. until now, if you owned an Ipod, you could only buy compatible music downloads for it from Itunes and vice versa - songs from Itunes will only work on an Ipod).
Second of all, the quality - Amazon files are encoded using Variable Bit Rate (VBR) with an average of 256kbps, where as Itunes' standard tracks are encoded at 128kbps constant bit rate (CBR) (the Itunes Plus tracks are encoded at 256kbps constant bit rate). The advantage of using VBR (Amazon) over CBR is that more data is used for the complex parts of the song while less is used for the simpler parts therefore improving the overall quality.
Also, as you have mentioned, songs from Amazon tend to be cheaper than those from Itunes.
The software Amazon uses to download the tracks is quite good and it automatically imports them into Itunes for you.
The only advantages of using Itunes in my opinion is that it has a superior search function and the 30 second previews are of a higher quality.