I really like dagstuhl - the organisation is wonderful - lots of lovely quietly smart tricks (for example, the librarian puts out copies of all the books by attendees for display and they are invited to sign them!) - the bar (wine) is fantastic (makes up for the so-so food - although the cheese platter is superb !) - the visitors book and other information for wider awareness of what is going on and who is there is excellent - the area is great - even the "interesting" challenge of getting here from the "nearest" airport or railway station is a fun problem
only 1 minor complaint - the website (in almost all ways very very good) has a facility for attendees to upload materials for their seminar/talk/wiki/discussion etc - but the site as a design flaw which makes it very counter intuitive to getting the first thing uploaded (when you navigate to My Documents, you MUST have a title and click on the very small "save all" button to avoid weird apache error messages:)
otherwise 999/100 for the best seminar/retreat/workshop site in Europe by miles
fine snooker room ,very nice music room, great cellar, good walks/hikes, pleasant and efficient (and mostly invisible!) staff....