It depends on what you want - power or portability. I've had/have Dell / HP's flagship mobile workstations and in terms of power they lag far behind what you can get on a desktop - but it depends on the level of intensity of what you're doing, and where you need to do it of course.
I use HP's Z800-series workstations as my primary workstations and main home computers. I can vouch for reliability, flexibility, support and stability.
http://www.hp.com/united-states/campaigns/workstations/z820_features.html
Dell's Precision range is indeed as the above poster says very good bang for the buck especially if you call them they'll drop their pants in a flash, but I find the construction of a noticeably poorer quality than HP's and historically I've found QA is far more variable, though support (with the paid-for Prosupport - I never rely on 'free' warranty, even if a 3-year one is included) is very good overall.
I'm currently trialling a Dell
M4700, Lenovo
W530 and HP
8570W (my 17"'s will stay HP, and I have a few
8760W's) to choose my 15" 'more portable' mobile workstation, and am waiting for a
T7600 to arrive for testing, which I'll face off against one of my new Z820's. As much as I like the HP gear, I have to acknowledge that I can buy several more Dell workstations than HP of essentially the same spec where it counts for the same money. I run my own remote and local workstation clusters and private cloud, and it's not like I'm some Russian billionaire so the cost savings are
very tempting - but I won't do so at the expense of reliability (unless I have no choice, as in e.g. the case of the relatively speaking POS Mac Pro's on which I have to run OS X software).
There are lower-end machines available in all of those ranges which may fit your specific needs better - the HP
Z420 or Dell T3600 / T5600 comes to mind in terms of desktops for a lower-level user - but my approach is only to buy flagships unless there are specific niche requirements - YMMV.
In terms of desktops, a lot of people who don't actually use this level of power for a living might say 'build your own' but if you're actually using this as a tool then it makes far more sense to buy a good machine.