I looked at MonetDB as part of researching column databases in general. It was as fast as the commercial databases on some queries, I was really impressed. The main thing that put me off was that you couldn't seem to nudge data layout / the query optimizer to do exactly what you specify. They seemed to have the idea of an all-knowing system that would optimize for you, perhaps as a research idea that works but for commercial use it worried me too much.

For anyone interested in column databases in general I put this list of comparisons together: http://www.timestored.com/time-series-data/column-oriented-d...