For what it's worth . . .
HP laser printers have been rock solid for as long as I can remember. Ink jets have had problems, most widespread being driver problems. The lasers have started a downward turn in the last couple years as HP tries to make a price point, but they are still built better than most and are by far the easiest to service.
HP computers are another story. HP PCs have always been a low end POS. A new HP PC will probably provide joy when new and does offer a price point. Longer term, they tend to be more trouble. Components are often HP spec, air movement is not handled well and the power bus is not well designed. When it breaks, throw it away. If you were dealing with a high end server, the story would be different, HP does OK there but still doesn't offer a good value.
The merger of Compaq into the HP mix has not really helped, they had simlilar problems on the low end, with the added frustration of proprietary hardware and bios that further complicates repairs/reconfiguration. Again, the server products are not a problem, in fact I generally prefer their servers. Not sure how the problems are being merged at the present time, haven't dealt with the low end there since the merger, but it can't be good.
On the Dell side, I think you will generally get a good value. No problems with venting/power/proprietary hardware. Cases have gone downhill in recent years, but not to the point that I would rule them out. Recent lead times have not been too bad. Exception would be if you ask for a component that has and extended lead time, but they will usually point out if that is the problem. Service could be better, but I don't think you will get better out of the others in that market.
$400 is a good difference, but it has to come from somewhere.
It was mentioned that you could build the same machine for less, don't think you could buy components for what the Dell will cost you. Retail OS alone can be a killer. You would end up with a better machine that way, certainly a better cabinet, but you won't get a cheaper machine.
Short version, If you need a laser printer, call HP (inkjets call Epson, dot matrix call Okidata, for what it's worth). If you are looking for a $15K+ server, consider HP. Right now, you need to call Dell.
