The tuner pedal is a pedal that I think is necessary for every bassist, especially if they are building pedal boards. It might not be the most fun pedal, but it is probably the most important pedal out there, as without it, you'll sound terrible. Korg's entry level Pitchblack is a great way to have something to keep you in tune for not much money. Used examples can be found for cheap; I paid $40 used for mine, and it is worth it.
First, the good. The display is nice and bright as well as being easy to read. There are various modes that are accessed near the power jacks, controlled by recessed buttons. These include the standard "needle mode", two strobe modes, and a mirror mode. Personally I just leave it on the standard mode as that is what I am most accustomed to. Accessing the mode button (as well as the calibration button) however can be a bit of a pain however if you have mounted the pedal to a board, so adjusting modes on the fly is not really necessary. Sensitivity is also good, with the tuner picking up the string quickly, though more on this further down. The pedal is also true bypass, and even better, can power a second pedal (or more if the mA reading of every combined pedal doesn't exceed 200mA). This is good if you are running low on power supply outputs, or if you are just powering some simple drive pedals.
There are a few drawbacks. As others have pointed out the pedal does have issues sensing anything lower than a low D, though on my LTD this is less of an issue than on my Phoenix. That might be down to the fact that the LTD has a MM type bridge pickup and the Phoenix has a single coil (I usually tune using the bridge pickup). There is also the aforementioned issue with accessing the mode and calibration buttons. Build quality is good; it isn't tank-like like any Boss pedal but it's rugged enough.
On the whole I think this is a great tuner pedal, especially if you can find one cheap. Good construction, ease of use, and the clear display make this pedal a useful tool in your arsenal.