I have a Snark. It's... okay. I know they're very popular so maybe I got a bad one, or else it just went bad or something, but it just doesn't work very well anymore. I got it about 4 years ago. It's hard to get it to pick up the vibrations and I always have to fiddle with it, so I don't use it much anymore. I remember that I was never a big fan of it to begin with. It was always a little finicky. I was happier when I had a regular Korg tuner before that one, that listens to the sound of the guitar. The only benefit I see to the clip on tuners is that you can use them at a gig where it's loud and you can't use the ones with the built-in microphones.
What I use for my electric guitar is the built-in tuner in my Eleven Rack. I think it's quite accurate, and suspect that there are some great pedal tuners you could get that would also be very accurate. For acoustic guitar I use a MetroPitch. I bought it as a metronome, but the tuner works great as well. With those sort of tuners, I don't think you get the same level of accuracy, so you have to use your ears as well. It also has a tone generator which can be used to tune by ear.