I'm typing this on an m15X
I went a little spend crazy and bought an m15X, it's a great laptop although it does have some faults, like the track pad which is integrated into the surface of the casing. It's hard to use because it's the same shiny material as the rest of the case, your finger doesn't glide smoothly over shiny plastic. You also can't feel when you've reached the edge of the trackpad and in fact the actual sensitive area ends before you reach the edges, making it even harder to learn how to use.
The touch sensitive volume control is also hard to use and varies wildly in how well it works, sometimes deafening you with a sudden jump in volume. On the subject of volume, since it's Vista with individual volume controls for each application the touch volume control only changes the volume of the current app. This is useless if you want to turn up the overall system volume when you're in a game. Turning the game's volume up to full is useless if the main volume is still set to a whisper!
IIRC Alienware only offered 32-bit Vista, which means the 4GB of RAM they'll sell you will be a gigabyte short too...
It might sound like I hate the laptop, but I don't, if you're a gamer it's excellent for games, it's incredibly powerful and very fast. Of the games I've tried so far (Gears of War, Assassin's Creed, Call of Duty 4, Bioshock, Sega Rally, Portal) all run with everything on maximum. It also gets over 10,000 in 3DMark06 if you're into that sort of thing. I haven't tried Crysis, but I'm sure it would make a decent job of it.
I went for the 1440x900 screen as it has the shiny X-Black type finish that gives a much nicer picture. The hi-res screen Alienware offer doesn't have that, plus at 15" I think the hi-res screen might be hard to read. The m15X also has a non-MatSHITa DVD drive, so DVD Region Free works again (my last laptop was cursed with a Matshita drive meaning I couldn't watch any of my Region 1 DVDs).
It also has the latest MXM slot so when the next generation of laptop graphics cards comes out it should be possible to do an upgrade. If you're a gamer, the laptop is definitely worth it, but if you don't need the horsepower for games, then you don't need anything near as powerful as this.