While the "squeaky wheel gets the grease" excuses are all well and good, they don't tell the whole story. Sure, only customers who have problems complain, but if you're interested in customer service, what you really want to know isn't "Are they good people" but "Relative to other companies, what do they give me when thing go wrong?"
I bought a Sentia because I was tired of the video-card problems I was having with my Dell Inspiron 8500. Five times over two years, I'd had to call tech support and ask a tech to come to my place. Five times they'd replaced either the motherboard, video card or LCD. (Sometimes they sent the tech out with all three, just to be sure.) I figured I'd never have the same problem with Alienware.
I was right: I didn't have the same problem. When I encountered similar video card failures with Alienware, I found out that the "onsite" support I'd paid for wasn't really onsite at all. Any significant problems I had--i.e. any hardware failure involving replacement parts beyond memory and such--would involve sending the system back to Alienware. Now my computer is winging its way to Florida, and I'm back to using my 8500. (For which, it should be noted, I just extended the warranty.) By all reports from their forums, I may not get it back for several weeks.
Is Alienware a great company? Who knows. Are many people happy? Sure. When you're making your buying decision, just keep in mind that "onsite" doesn't have its standard English meaning, and that if anything does go wrong, you're going to get real familiar with your local Fedex delivery person.
Check the forums. See how long you'll be out a machine if something breaks down. You're talking about a Sentia, so you're buying a business/school machine, not a gaming rig. That means it may be a critical system, and what you worry about with those is downtime. I've already had more of that with this one issue than I've had with three years of Dell, simply because when my Dell was down, I didn't have to wait around for shipping.
So there's my advice. If you absolutely need the machine working on your desk, go elsewhere. Sure, many people will be happy, but at the end of the day, you don't need the risk. I don't say that because I hate Alienware: I *love* the machine. But I would have loved not taking time from my exams to deal with technical support more. In the end, however much I love it, it's just too much cost.