only bad thing with my dell (5150) is the nonstandard case and motherboard, which means no replacements. However, on the plus side, the case is very nice and easy to work in.
Other than that they are cheap, up to date specs, customer service isnt too bad, and you get a good deal of bundled software.
Bit of a pain in the ass to change the PSu in one (they can take standard 24 pin ATX power supplies, but they are a bit bigger than the stock dell one, so its tight (i think one of those slightly smaller psus designed for micro atx cases may be better). However, for most people, the stock dell one isnt bad (its 350w, but seems to be very efficient, never had any problems with it ever. my 2006 pentium 4 model and has dual 12v rails - wouldnt like to try and run an 8800 off it, but it ran my x1650 pro just fine)
Also, they wont take graphics cards that are dual slot.
But for most people (non gamers/modders) they are great. Apart from those mentioned above, all the other components are still easy to upgrade.
Bad point though, is the bios doesnt allow overclock etc....
Good point is that dell offers actual windows media.