I used Nirvana several times back before they updated everything for the pH Perfect line. I was very impressed with it then, but I typically just used Big Bud which I feel edged it out slightly in performance. Of course Big Bud isn't organic, and Nirvana is, so there's that.
Their new pH Perfect line has it as part of the Professional bundle level, which I'm almost definitely going to give a whirl on my next grow. Depends on the budget, of course. I'm very curious how well it will do in the new version.