This is overall an okay tuner. It does well for the price, but here is what I didn't like:

The worst issue for me (as a piano tuner) is that this device, even with a great external mic connected, does not have a good 88-key range as it guaranteed. I really have to tune by ear for the last entire octave on the high and low ends without the mic, and it isn't much better with the one. The good news is that the entire mid range is a lot more accurate with the mic than without.

2nd issue: It has some issues tuning in a room with any other noise at all. I live in a house with a well pump, and while my piano is five times louder than the well pump, it gets severely distracted by it to the point that I can't tune with it. So I can't tune my piano without stopping for the well pump every 10-20 minutes. I have used a Yamaha strobe tuner that can keep focus in a room with someone stringing a piano, tuning, machines running, etc...

The other issue I have is not something that Peterson said it could do, but I would like to see a version of this that actually displayed how many cents sharp or flat you are. The strobe wheel is great, but knowing how many cents off I am is pretty useful (if I want to tune a piano that I just strung 10 cents sharp or something...)

I would keep this as a secondary tuner, but if you tune for a living or just do it a lot for that matter, I might spring for something more expensive. Good enough for the price I paid.