I thought the Keller book warns cooks that +/- a couple of degrees can result in disaster, but the engineer in me refuses to believe that. Maybe it's just the fish dishes that matter.
A couple degrees either way aren't going to screw up beef, pork, or chicken. If you aim low, it won't screw up fish. If you have a very specific egg doneness target (and eggs are one of the very few things I'm better on a skillet with than in SV), one or two degrees matters a whole lot.
And finally, I think one of the big places temperature control matters is with vegetable cookery; there are SV applications that aim to get in between the temperature at which starches break down and the temperatures pectins break down, and if you're trying to do that (I guess to make watermelons that have the mouthfeel of filet), maybe the PID controller is a dealbreaker.
But if I could find another medium to use instead of water it would be nice.