I have one of those Rock BR (18"), got it about a month ago. I also have a Deadshot and Rock Deluxe, all by Caldwell. (Started with the Deluxe and kept on moving up.)
You can see and compare all three on one page from Midway's site:
http://www.midwayusa.com/ebrowse.exe/browse?TabID=4&Categoryid=15820&categorystring=655***689***
IMO, the BR is a pretty good rest for the price you pay. I haven't, though, had a chance to use or even see first hand products from Sinclair or other higher end manufacturers.
The only shortcoming of the rest is the lateral fine adjustment. The top portion of the rest that moves for lateral movement pivots at one point at the front. The adjustments are made at the rear. To move left/right, the hand adjustable screw is turned that moves the rear portion of the rest platform. Since it pivots from a single point up front, the platform not only moves left/right but also turns slightly. This, though, should not be a great factor in fine tuning your aim. Aiming adjustment should be small enough to any negative effects wouldn't be seen from this shortcoming.
I think the reason for this is that it is a low cost method to provide a a solid (no excessive play) platform that is adjustable. More parts and machining are probably needed to make the plaform move truly lateraly and have no play.
Of the three rests I have (actually started with and still have a Hornady Delta Rest but haven't for years and probably will never again use that rest), I use the Deadshot for the simple reason that it is the easiest to tote around and no assembling/disassembling to do (I carry them around in a backpack). But for serious work, I'll use one of the other two.