Ive got 3 of the 1TBs in a Drobo, with a 640GB WD too. 1.5TB at the office. No problems yet, but the seagates are quite loud spinning up. The WD drives are much quieter and run much cooler.
I've been quite pleased with WD lately, (640GB is fantastic) but I think Im going to hold off on the upcoming 2TB drives, because they are the 'Green' variety, that throttles down RPM from 7200 to 5000 to save energy. Previous iterations of this model have had problems in Raid and Drobo, because they basically turn off too much. I'll wait for reviews, and confirmation that they work in Drobos. (The seagate 1.5tb was originally accepted in Drobo, and could be seen on the "drobulator" page, but they pulled it as an option after the raid problems surfaced).
The caviar blacks are what you want, imo. I figure WD will eventually offer a non-green model of the 2TB in caviar black format.