I made a post in favor of the Sonic Research Turbo Tuner a few weeks ago. Within a few days of that I entered a contest to win a new T.C. Electronics PoyTune and won. While I would never have gone out to purchase one since I had a few pedal tuners around, I did think that the concept was pretty interesting. I got it last week and it's pretty cool.
Besides reliability, basic appearance, and footprint size, I think most people compare tuners based on speed and accuracy. Regarding accuracy, while the Turbo Tuner and Peterson strobe tuners are considered the most accurate, I think most tuners will get you close. Your ears are the final criteria. Some folks will hear when a string is slightly out and some won't.
So that leaves you with speed to consider. Again, the Turbo Tuner is considered about the fastest. But that's tuning by checking each string, one at a time. I have found that when using the PolyTune at home it's really fast, because I pick up my guitar and strum once across the strings and see instantly that maybe only 1 or 2 strings need a tweak. So I only fine tune 2 strings instead of checking 6 and adjusting 2. Is it faster than other tuners when tuning 1 string at a time? No. But total time given to checking the tune of a reasonably well tuned guitar is definitely less.
So what I had thought what was probably a 'gimmicky' product before having first hand experience of it is actually at first impression a pretty cool tuner ~