I've been stalking the Vitamix for some time but haven't bought one yet. Today at Costco they were demo-ing the Blendtec and I really liked it. The guy made a few good selling points - Vitamix copied their design and had to pay a multi million dollar fine, Blendtec has a shorter container standard, Blendtec has the same container for wet and dry, Blendtec doesn't need a tamper, Blendtec has 1 more year warranty (7 vs. 8 I believe?). And the price is $329 for the version with pre-sets.
Why would I want the Vitamix over the Blendtec? I had barely even heard of Blendtec before today and now I'm very tempted.