I've done some quite serious testing with a fairly large spatial table (5m objects) and quite complex polygons for intersects and contains functions. Initially there were some quite serious bugs in terms of useability but Holyfoot has fixed these very quickly -- see
http://forge.mysql.com/wiki/GIS_Functions for details.
In my view, these functions are ready for some major testing now and any help getting these ready for inclusion in a future stable mysql release would be greatly appreciated.