I actually notice a pretty decent difference by turning it off... enough to where I'll disable it a lot of times.
Having said that, I seriously think there's a bug in the software somewhere regarding the GPS. Steve mentioned one time that it only gets turned on when needed and then turns off when not in use... yet it always manages to stay on. Disabling does not mess with the functionality of apps that use location data- all it does it make them ask again to use it, then it turns it back on.. but they never turn it back off when they're done, which is where I think the problem is