We have a Liberty CRD, had a few problems but nothing really out of the ordinary in over 100k miles. Between our TJ being sold and our JK being on order, we took a couple of trips in it that had some decent off-roading (nothing extreme, but fairly rough trails over passes, up to ghost towns, etc) and it did surprisingly well
Mostly I like the fact that with a small lift and aggressive tires I still get 21-24 mpg highway even with a lot of hills. And the torque is great. I wish there was a Diesel option for my Wrangler, even after re-gearing, with 35's and a rooftent, I could use the torque and a few extra mpg would be great for the extra range.
There are some pretty built Liberty's out there; the L.O.S.T. site is a good place to look.,but, as mentioned in another post, the lack of a solid front axle, among other things, makes anything really extreme fairly costly.