I think that this is very personal. Some people will buy a rack mount system for the best speed performance and some other person will buy a system for the best reliability.
Personally, when I build a rack mount server I always put Intel processor (Celeron or P4) and I make sure that the main board have a Intel chipset and not a SIS, VIA or any other different chipset. I think that it makes lot of difference on the reliability of the server.
I'm not really informed about the AMD reliability and motherboard performances but I think that a AMD can do a great job too. Just make sure to have a good quality main board and you'll be sure to get a reliable machine at the base.
The only place that I would use Rackable is if I was going to be doing a DC based solution. Also if I was doing this DC based solution I would look for something with rackable that I could evacuate all of the exhaust to atmosphere. The cost/density just doesn't get you there, even if your rent rates are in the levels of Manhattan's rent rates.
We did a pilot project with just 20 of their 1U dual Opteron dual core box. Potentially they're nice machines, but you need to be geared up specifically for managing 'compute' type servers -
Our take was that for Linux based deployment, where we could install them diskless (using iSCSI boot from SAN), and serial based management is sufficient they'd be great.
For Windows use we went with DL360g5's as the DL365 still hasn't shown up - built in RAID with hot swap drives, and iLO with remote GUI console make them worth the expense for us.
The big Linux project we had we are doing on VMware VI3 instead of physical machines, so Rackable became moot. NB. they don't stock machines either - it's all build to order, so plan your deployment timescale accordingly.