Came to this thread a bit tardy but...better late then never!
I think that the 3627 Merry Men's feast is one of the rarest sets, although so is the Museum, it seems.
Hard to find yes but a number of these come on ebay each month, i would say 3-4 sets of rmerry men and 1-2 museums ( own view of course ), the rollecoaster and that infamous dagger i have never seen on ebay ....
On the subject of rare sets , with reference to medieval which is by far my favourite theme the following is very rare also
Medieval towen folk and well 3487