Stone or Lenovo here. Haven't had any major problems with either, just the odd HDD here and there.
Lenovo is more of an enterprise product than Stone but you have to work harder to get good pricing on them (I find Lenovo are hopeless if left to their own devices but if you have an account manager from a big reseller like Misco on their case you're sorted). Also goes without saying to avoid the Edge series desktops as they're inferior quality to the M series machines (crap Realtek NIC etc)
Also another +1 for ICT Direct, got some M58p machines in over summer and they're fine :)
Personally wouldn't go near Dell, awful service and pricing. Hated their kit for years since the first machines I encountered with all the proprietary parts.
We are now all Dell with nearly 2000 machines over the last 6 years we have only had about 15 that were faulty and it was normally just a stick of ram so it means the machine is back in service in minutes.
For anyone who says Dells prices aren't competitive, are you dealing direct?? and if you are you need a new account manager as no one can normally touch dell on price/spec/warranty.
You also get great driver support that is fairly current and tested, so no hunting round for drivers and then troubleshooting bugs.
I aren't a Dell fanboy but if someone can beat 15 faulty computers out of 2000 i will happily take a look.