I own three Thule 530's which are attached to my Thule roof bars on my small hatchback (these are the the type where both wheels are left on)
I've carried everything on them from Roadie's through to 160mm full suspension & have used them for short trips aswell as further afield in the UK.
I haven't had an notable issues with any 'wobbling', the only time I had a minor issue was when I was driving along in a very strong crosswind, where a front wheel came slightly loose in it's binding. I do find I have to watch my speed a little, but can do 70mph with them fitted.
I have noticed a drop in fuel economy, but by comparison to the rear mounted rack I had previously the bikes are much more secure.
I also had the same issue with the frame being scratched as described by suggsey, but I heli-taped part of my downtube on all my bikes, which sorted the problem. I also slid a bit of inner tube over the clamp jaws.
In terms of fitting I have found that it can sometimes be a little difficult to clamp some full suspension frame designs (basically the more travel the harder they are to get to fit)
I would buy the same one again if I was starting out new or perhaps go for the next Thule model up (591), which I've been told is a bit of a better fit for full suspension