I've had one a couple of years - but with the SRAM rival groupset.
I posted a review shortly after I got it.....
http://www.cyclechat.net/threads/planet-x-rt-57-rival.115175/
The RT-57 is a cracking bike. Handles fantastically and easily the match of anything at anywhere near the same price.
However there are a couple of things to be aware of:
The frame geometry is fairly racy. It promotes a longer, stretched out riding position which may (or may not) be what you want.
There's bugger-all clearance on the frame. You can run 23mm tyres, but nothing bigger, and dont even think about mudguards ---- not going to happen.
The front end (and indeed the whole frame) is incredibly stiff. It's not a boneshaker by any standards, but the standard Planet-X Strava bars (again very stiff) transmit every road lump straight through to your arms. Personally the bike suits me very well and I've done the coast to coast in a day twice and numerous other 100+ mile ride with no i'll effects. The RT58 is a similar bike, but with a more relaxed geometry if the above bothers you......
Planet X were a pleasure to deal with, and I've since used them multiple times with no issues.