I've got a pair of Thule 591s, but have never been completely happy with them despite thousands of miles of trouble free use. I now mainly use the Halfords rip off of the 530 for our two bikes. The clamp either side just feels more secure - the bikes don't move at all, whereas they do wobble a bit side to side on the 591s, plus the 591s clash a bit with where the downtube cable guide goes on my Canyon.
I'm just happier with the simpler, more bombproof rack. The genuine Thule version continued to hold my Marin years ago when a freak cyclone blew the entire roofrack off (seriously, the paint was stripped to the metal in 3" strips where the rack was ripped off)!
The cheap rack is not nearly as easily adjustable, but there is no other downside that I can think of, and they are much, much cheaper. I run a 591 in the middle for 'visiting' bikes, although it whistled until I duck taped the main mount.
HTH