We use GPS (trimble) pretty heavily where I work. As far as when it goes down, what do you do... if it's not the final pass, it's generally not hard to "rough in" grade till it does. Most of our dozer guys have a good enough idea of what they are doing in a particular location, that if it does go down, they have a grace period of work they can still do without grades. That being said, I've never personally seen it go down for longer than 15 minutes. Get into trees or near building and you'll have more problems with getting a good satellite signal, but that's a "known", and hopefully then you'll get survey to pop in a few stakes.
Where GPS can get annoying and a little ineffective is when the guys that make that particular plan, don't go over it. Job I was on last year was a four mile road, and was cut into 3 design plans, where two of the plans came together no one bothered to make sure that the final plan vs. the E&S plan was changed and that resulted in a "bust" of a three foot speed bump. Fortunately, I've got just enough brain cells to be able to use the bubble, till they got around to fixing it. Ironically the total station that the grader ran off of was different (and correct) in a few spot by as much as 6".
I like GPS, but it like everything else has it's short comings, most seem to come from human error, either the one in the field or the one in the office.