Granted, Toyota have been a massive failure, especially considering that only Ferrari have spent more than them over the duration.
Still, if you look at those who have left and Renault, the record is impressive:
Honda - More or less financed the development of the 2009 Brawn GP car, also had a championship-level end to the 2006 season with Button.
BWM - 16 podiums and 1 win, 2nd in 2007 and 3rd in 2008 in constructors championship.
Renault - 2 world drivers' championships for Alonso, as well as 2 team titles, developed the revoluntionary (and subsequently banned) mass damper system.
Toyota - Undoubtedly a failure, but there were signs that it could have worked when Gascoyne was there (4th in '05), sacked for making statements of micromanagement by heads.
