Had my Dell Inspiron 9400 (one step below the XPS in performance terms, but still great) for 8 months now - no problems to speak of yet. Mate of mine with an XPS has had no problems either. Built quality is generally good, though they're not the most portable of systems (big 17 inch screen, weigh a ton). Definitely a desktop replacement rather than a true laptop.
On the buying side of things - always haggle them on the sales line. Try to get upgrades or accessories free of charge. My system spec'ed at £1400 cost me £1200 after a bit of haggling, with a laptop bag and internal bluetooth module thrown in.
As for alternatives, I'd recommend Rock Direct - they specialise in performance laptops, and have a very good reputation.