It's difficult to say. ASUS, MSI, and Lenovo generally have very similarly-priced laptops with similar specs at lower price ranges, but they diverge at higher ones. Gigabyte is a new player in the gaming laptop field and have the best-priced gaming laptop right now, a $1300 powerhouse with a Broadwell i7 and a GTX 970M, but they are not exactly proven in terms of build quality (though they are among market leaders in the motherboard and GPU board industries).
The best answer is that it depends on the price range, as each company has something unique to offer.
2) If money is no object, the Razer gaming laptops seem to get high marks for build quality.
3) In general I'd say Asus and MSI have good fit and finish. Lenovo is so-so, but their support is a lot better (expect about a month turnaround time for warranty service with the other brands). Gigabyte is new and seems to have a lot of quality control issues right now. (My gaming laptop is a Gigabyte, and I've had few problems. But based on the owner forums I visit, I seem to be the exception.) Clevo (Sager) absolutely give you the most value for your money, but quality is iffy and a lot of people think they look ugly (myself included).
4) "Worth their price" is more a function of how old the gaming laptop is. The depreciation on gaming laptops will make your head spin. I paid $1800 for mine, and it's already down to about $1400 after a half year. The updated version (with Broadwell CPU) is currently about $1500. So if you want the latest features, you will pay out the nose for it. If you're willing to wait a half year or go with last year's technology, you will find much more reasonable prices.
It depends on what kind of gaming laptop you are after.
At the low end acers are generally thought as good budget end gaming laptops.
At the mid end, asus gl series, msi ge series.
At the high end, asus g series, msi dominator series or any barebone laptops