True bypass is very helpful for certain pedals. Fuzzes and Wahs at the front of the signal path are quite sensitive to your playing. A Buffered pedal before them cancels all of that interplay. Some do not notice, some do. There is much hoopla made about true bypass, like it is the next reincarnation, but that is marketing. As has been noted, it has its place and is good when used appropriately. For homebrew pedals, it is a heck of a lot easier to wire. For production pedals it is way too expensive for the switch, and the buffered transistor version is much cheaper. As for what is true bypass, it is truly the switch that is on or off with the pedal. There are various shades of gray for intermediate versions of the switch (the Millenium bypass for example) that use electronics to do the same thing. I don't get the whole over/under rated concept, but if you know what your bypass setup is and where it is appropriate to use the different versions, it will guide you on getting the most out of your gear.
Patrick