I have 3 Dells in my house currently and have owned or used (at work) probably 5 or 6 more on a daily basis. I have had exactly one problem with one of them. The hard drive that came with the latest one was bad. They had a guy at my house the very next day I reported it with a replacement. End of problem on my end.
My biggest complaint to date is that my laptop was a little too heavy, but this was 3 or 4 years ago. But I think that was pretty much industry-wide.
Building your own computer may be the best deal for someone with good to above average skills, but for the most part it would be way more hassle than it's worth.
Whoever said stay away from Gateway gave the best advice so far in this thread.