I have previously posted on this thread that I have used OxbridgeEssays (amongst others) and they were fine - which they were.
I have also used ukessays.com and for two out of three papers they were fine - good 2:1 papers - the third paper was well written but I later found out that the statistics were inaccurate which invalidated the argument presented in the paper.
I have used paperstore.com and found them appalling. The paper was a poor standard, failed to address the question, doesn't use the source I had asked for at all, and their customer service was dire - the promised 24 rewrite arrived a week late and the standard was still unacceptable.
Junglepage.com were okayish. The paper was a little bit basic in coverage of the topic and it arrived a day late but it was a big improvement on the paperstore.
Custompapers.com were good - I've only used them for once so far but the standard was high; it was easily a good 2:1 standard and was an equivalent standard of the two good ukessays.com papers and the oxbridgeessays.com papers. Custompapers also delivered 12 hours before the deadline which was unexpected and much appreciated.
Given that the output of custompapers.com meets a good British 2:1, I will be using custompapers over the next academic year. They are far cheaper (£187/$300 for a 3,000 word essay versus £300/$480 at oxbridgeessays.com) and from what I've seen the quality at Custompapers is comparable.
As Anamal says, you might want to have some reassurance that an "expert" at an Oxbridge university has written your paper but that reassurance obviously comes with a hefty price tag. I'm happy with getting the same quality without the label and having money left in my pocket!!! That said, if Custompapers couldn't meet the deadline or cover the topic, then I would use ukessays.com or oxbridgeessays.com again but in my opinion custompapers wins hands down on quality and price.