I personally (as opposed to reasons from our organization) want to upgrade for two reasons: 1) I think the 2005 performance or perhaps just our implementation of it has been flakey. I'm hoping we will get rid of the weirdness with 2008. 2) I'm very interested in the new T-SQL options such as the MERGE statement and the files data type. I have a need for both.
Is R2 expected to be any more "stable" than plain old 2008? (Or is that called R1? I don't like this naming convention. I think a new version should get a real new name. <side rant>