I just finished reading Variable Star, a semi-posthumous collaboration between Heinlein and Robinson. On a (personally biased) two-axis scale where Orphans in the Sky is a 10,0 and Lady Slings the Booze is a 0,10 I'd rate it a 4,5. Passable by either classification, but nowhere close to my favorite solo works by either author.
(
Warning: Possible Spoilers )