Laptops are good for surfing the net or writing term papers. They are limited for games. Dollar for dollar, the keyboard will usually not be as responsive, the display not as good, the internal hardware not as powerful, overclocking limited, and you risk it all with overheating issues. If a part breaks down you won't be able to replace it like on a PC. If a part turns out to be a bottleneck in game performance, you won't be able to replace it.
These are all substantial disadvantages you suffer for better portability and convenience. If you must play the latest military shooter on low or medium settings at your local coffee shop, then be my guest.
I've used a laptop for gaming as a backup between PCs and the answer to laptop gaming is: no, just no. Unless there is no other choice, and even then it is a toss up because it is always going to be a gimped experience in all but the most basic games.
Also--agree with never buying an Alienware, mine was utter crap. Yes, I made that mistake one time. They didn't set up the sound right and their proprietary power supply they kept bragging about fried within six months. Stay away from pre-fabs. Research the parts and build it yourself for the greatest joy. It will be a hassle but worth it in the long run.