I use this with a Sansa Clip+ 8GB. It charges the Clip+ using the USB cable supplied with the MP3 player and it even allows the Clip+ to play while it's charging, with no ill-effects on the music. I've even used it with a USB extension cable and it works fine (as you'd expect, but I had to check).
I now use this to power my Clip+ while it's playing through my HiFi at home - as well as charging it when away from home (e.g. on the train) when I don't have or don't want to boot up my laptop.
Considering there are other items at twice the price or more that don't do any more than this, you might wonder where the extra money goes. Well, this plug/adapter is well made enough, but it's hardly luxury finish. But there again, how often to closely examine wall plugs? The USB socket could possibly have a tighter fit when I use my USB extender, but it's not as though it doesn't hold it well enough and it still works perfectly OK. And don't expect any fancy retail packaging (that you'd only recycle anyway) or any instructions -- it's obvious how you use it!
Overall I'm extremely pleased I didn't spend that extra money getting something a lot more pricey. Like I say, this does exactly what you need at a very low price.