I think they’re great! But then I own one that I got 2nd hand a couple of years back for a trip to the Alps. The plan was to sell it afterwards, but I kept it and it’s off on it’s 3rd Alps trip this summer.
Lots of people told me it would kill me as it wasn’t slack enough, only had QR wheels and forks etc. but I’ve managed to ride at the same pace as friends on more expensive and modern bikes and haven’t killed myself yet.
2007 was a transition year in the Marin Quad Tara range as they went from one rear suspension designs to another newer chunkier version. The bikes in the years before had 4-6″ adjustable rear suspension, this was fixed on the Rock Springs in 2007 at 6″ and it’s geometry is slightly slacker due to the use of an Alpine Link in the suspension. Most of the other Quad Tara bikes used the new suspension design so worth checking whether it’s a start of 2007 ‘old shape’ or a end of 2007 ‘new shape’, mines the ‘old shape’
Hope that helps, good luck.