I REALLY like my X-Challenge. It's relativelylight, makes good power and has been reliable. The air shock wasn't terrible, but it had weird characteristics. I have to think the one on the HP2 is better. The lack of rebound damping makes it feel like it wants to launch you off the bike and fully extends the rear suspension under heavy braking. But it has been replaced with a Hyperpro now, and the difference is awesome. The aftermarket is fairly limited and expensive. Adding fuel capacity is real expensive, but with mild fabrication skills several of us have added aux KTM tanks to get ~200 mile range. Mine is setup more as a light adv. tourer with racks and soft bags. But it still probably weighs less than a stock DR650 with lots more power and FI.
I've never ridden the TE models, but they seem to be geared a little more toward dirt, albeit slightly. At least the 610, didn't the 630 gain some weight? They seem pretty similar in mission though. I might give the edge to the BMW in overall reliability, but I think both are very good - but not on par with something like a DR650. But the Rotax single seem to run forever.
I doubt either was worth the ~$8000 they were originally priced at. But both dropped to about $6000, which I think is a bargain compared to the price of a new DR (which I think is a great bike). I think history has been more kind the the X-challenge than the market was. 6 years later, you'll still pay about $5000 for a good used one. On the plus side, it will likely be well sorted. I've had 4 motorcycles since I bought the X, but the X always stays.
But with all that said, the Husky Terra and Strada should certainly deserve a look. They're basically X-challenges without the quirks, but smoother and 5 extra HP (but 50 extra pounds). If I totaled my X, I'd either buy another or grab a Terra.
