I made a pumpkin dubbel last year as like my third brew ever.
Wasn't a big fan, though it definitely held true to the whole "the last beer is the best" adage. The longer the bottles sat, the better each successive one tasted.
I think the spices we used were our biggest downfall. The cinnamon and fairly robust carbonation made me think "pepsi" for the first 6 months or so.
FYI, it was based on the extract dubbel recipe in Papazian's book.