Comparisons with pickups depend on the position and type of pickup - after all, there's a world of difference between a bridge single-coil and a neck 'bucker. There is more difference from the magnetics than just the tone though. The dynamic range of piezos is much wider (play softly, the magnetics dominate, play hard and the piezos pop out of the blend). Also, the attack is much faster and the frequency range has both an extended high and low end.
It's not a true acoustic sound (not enough body and resonance in the midrange), but naturally it's almost exactly what I will EQ an acoustic to when blending with an electric guitar in a pop/rock mix. I usually scoop out the lower mids a bit from the piezos to clear space for the electric, but otherwise, the two sounds complement each other well.
Many of my comp entries have piezo mixed in to the electric sound:
I have a series of articles with clips up at
http://ratcliffe.co.za/articles/piezo3.shtml and
http://ratcliffe.co.za/articles/piezo4.shtml. Or if you just want to browse the clips without explanations
http://ratcliffe.co.za/music/piezo/. These clips aren't the best tone, but the differences are pretty clear.
Also delayed is :
http://ratcliffe.co.za/music/guitarforum/skanky_thing.mp3, you can hear the extended low end in the percussion (you can also minimize the low "thump" with your mixer's highpass filter if you don't want it). The piezos are the exact same take as the electric rhythm.
When I get a chance I'll record something with hard panning and minimal effects. Don't hold your breath this week though.
