As far as stomp pedal tuners go, the boss TU series is essentially the industry standard. Not an enormous question as to why either. A tuner pedal has one job, accurately display your guitar tune. Boss has always been a trusted and reliable company and this pedal is a must have for guitarists. 

Like most most tuners the pedal cuts the tone and displays the pitch at which your string is tuned to. What you can expect from boss, which I can't say for other tuners, is accuracy in tone recognition. Other pedals have a tendency to fluctuate and jump around on the display, poorly made inputs. You won't have this problem with the Boss TU-3.

Along with that you get the durability of the pedal, which is one of the most durable stomp boxes you can get. Along with that you get the support of Boss's five year limited warranty. Although the pedal is priced a little higher than similar tuners, there's one reason I wouldn't use anything else, the power source. With the TU-3 you can power all of your other pedals with a daisy chain. You will only need one power supply and you can daisy the power going into you TU-3 to all of you other pedals. There are two reasons why this is so important. Not only are you saving space in your rig and on outlets, which can sometime be hard to come by on stage, but many pedals do not come with a power supply at all. So instead of spending anywhere from $10-20 on power supplies for each pedal, you can buy one daisy chain for half that.