Dell's are fine if you like limited upgrade capacity. Have had some dodgy ones, and their software is annoying, but overall not a terrible experience. If you're the tech savvy type, I would recommend speccing or building your own though.
Dell consumer desktops - ie Dimensions - have same upgrade potential as most DIY systems. Corporate Optiplex don't, as often small form factor.
I think the days of DIY builds is nearly over, having being dying a slow death for some time. Its simply cheaper to go out and buy a prebuilt one. Generally, most prebuilts will be quieter as well...
Like many others, I have built hundreds, probably thousands of PCs, and twenty years ago there was good money in doing so. Alas now, I recommend most people call up the likes of Dell as they are much cheaper than I could ever be. I make my money in the setting up and bespoke software now (apart from my real job of supporting big servers that ISPs use)