I use them a lot. The quality is OK, but that depends on what you are using it for. I tend to go for the 4* rate, ie the best of the cheap options. My experience is that 2* level is pretty crap whilst the 5* level can be a bit pricey if you want a few articles that are around the 1,000 word mark. So you basically get what you pay for.
I have used the 5* option for important stuff and have always had a good experience. But I've had a few howlers in the cheaper options, but mostly, say 85% of the time, they have been great. You get the option of a rewrite if its rubbish and I've only ever had to do a total reject on a couple of pieces.
I tend to have between two and ten articles at a time and they are usually completed in two days.
Worth making a note of any really good authors and going direct when suitable.
HTH
BTW - what do you mean by spinning?