I have a madone 5.2 perf for 6 months now.
£2000 is a good price (RRP 2500), the 2010 models are more expensive. For that cash there are a few options available to you, so consider carefully but the 56 size is the most popular so will go fast.
I have been very satisfied with mine; it seems to me to have the perfect combination of comfort and response. The Ultegra SL is as smooth as DA; only slightly heavier. The weakpoints are the wheels and saddle - having said that the wheels are fine for general use and going strong for me.
At this price I would also be seriously tempted by a planet-x titanium/SRAM Red. However you won't be dissapointed with the madone.
A pro for me was the fact that this frame won the '07 TdF, but that may be irrelevant for you (or turn you off )