My wife has a T30 (old shape) X-Trail. It's slightly newer than the ones you're talking about (early 2007 car). We've had it 5 years now and she is very fond of it. Personally, I'm less fond. It's our first Jap car and it as well and truly shattered any illusions I might have had about Japanese build quality! In fact, we've spent more money on it than on her previous TWO cars put together (and they were both Alfas)! Brakes are the main problem. Famous for seizing. relatively easy to work on though, and relatively cheap to source recon. callipers. ALL T30 models have weak intercoolers. The end tanks are crimped on and after a while the crimps let go and they start leaking. Easy to spot when they do, and very easy to change. (Just did ours a few weeks ago, in fact). Early turbos (the sort of age you're taking about) ad weak bearings, but I think most will have been changed by now. I believe there was a recall. Our turbo has been fine. Exhaust gas recirculation valves are known to soot-up, but are relatively easy to access and clean. Some of the suspension bits are a bit fragile -anti-roll bar bushes and drop links in particular. Front wheel bearings also have a poor reputation (although ours is coming up to 90,000 miles and they're fine). Timing chains and exhaust servo vacuum pump chains (yes, it's chain driven!) can be a problem. I think ours might be starting to rattle a bit now.
That's the bad stuff. There is also good stuff. Even in "Aventura" spec, they're a good no-nonsense car. You can get rubber footwell mats and plastic boot liners and they're great for muddy kids. Practical and durable interiors in general, in fact. Best towcar we've ever had (very stable and loads of low-down grunt from the 2.2 diesel). Also superb in snow and ice. Less good on mud and deep ruts (no ground clearance). Basically, don't go thinking it's a Landrover! Up to a point, you can improve offroad performance with a lift kit and more aggressive tyres, but you compromise the on-rod handling somewhat. Remember they have no low-ratio gears and no diff locks. That said, the electronic traction control does a pretty good job. BEWARE, they did make a few 2 wheel drive early models!
For your long motorway runs, they're quite a nice, lazy drive. The suspension is fairly soggy and they're heavy, so they are quite comfy. Get the higher spec cars for nice heated leather seats and cruise control. We get about 32-24 to the gallon on the motorway (depending on how close to the speed limit you care to drive). Much over 80 and you'll be lucky to get much over 30 to the gallon.
Best thing about the X-Trail is the very friendly and informative forum:
http://www.x-trail-uk.co.uk/
If you get one with 17" wheels, the tyre choice will be limited and they can be quite expensive.