I'm monitoring about 2000 machines currently, with my production box running Big Brother + bbgen, and the test server running Hobbit. The load on the Hobbit box is about 1/4 of the load on the BB box.
Correction - since I just found a bug that caused Hobbit to use much more CPU time than it should:
The load on the Hobbit box is about 1/10 of the load on the BB box.
thanks, chris