Here are some reasons, from the last year:
1) Their small form factor PCs, we've had at least 25 pass through our hands, every single one of them has a burned out motherboard/processor because they cannot cool them properly. After several calls with their engineers, they basically would never admit to the problem being on their end. Oh well, they're the ones sending replacement parts to me every week for them
2) The video cards based on NV chipsets stop producing digital signal after a random amount of time on all their GX260's and GX270's. I worked for over 3 months with dell engineers who admitted that there was a widespread problem after I personally called in over 50 different cases of this. Their 'fix' worked about 1 in 6 times (and also took them 2 months to produce) and the other times new cards had to be set in place
3) Their smaller tower versions of the optiplex series have random leaking capacitors and power supply problems. I personally just replaced over 10 machines all from the same cluster, all with leaking caps. Their wonderful CS also would not let me give her the 10 service tags at once, had to do them individually, over two hours wasted on the phone.
4) The power supplies in the GX280's randomly crap out for no reason, at least 20 in the last 3 months.
5) Their dimension series is a joke, I won't even bother commenting on those.
6) Their laptops, some much better than others, are surprisingly error free, aside from the dead complete POS hard drives they ship with and the random overheating and shutting off after only being on for 5 minutes.
I could go on, but it makes me depressed just thinking about it. My job isn't primarily working with hardware so I'm sure the hardware guys could go on and on as well. I deal mainly with computer repair and building/support and OS/networking.
edit: And for my boy Zim, I would not recommend buying a PC from anywhere like HP, gateway, dell, etc. Find a local computer shop, research prices on your own, or find a friend that knows how to build them and BUILD YOUR OWN. You cannot find a big place that puts good hardware in their PCs anymore, if I've learned anything in the last 2 years it's that simple fact.