It's an awesome preamp. So much power at your fingertips but not at all overwhelming once you get the hang of it. I picked it out as sort of a poor man's Alembic preamp, and it fits the bill pretty nicely - it's similar in function but gets more of a conventional tone than the distinctive tone of their filters.
As far as it's capabilities it's great, but somewhat hard to put into words - You've got a sweepable bass or mid boost, and a sweepable high boost to go along wtih the standard volume/pickup-blend and optional tone control. I usually find myself boosting the mids, and adding a touch of relatively full-range highs, but sometimes I'll dial in Phil's 80's/90's EMG tone by boosting the low end, leaving the mids scooped, and boosting the very high highs.
Edit - Looks like the low pass filter goes from sub-audible up to 6.3khz and the high pass filter goes all the way from 1kHz on up. So they actually overlap a little!
Here's a very long thread with lots of info and some good sound clips:
Lots of my sound clips are here: http://www.archive.org/details/Fennario
Any show from Nov 2008 on features this bass almost 95% of the time. I use Bartolini soapbars, D'Addario Chromes in the earlier shows and Half Rounds in the later ones, and play with a pick except for the JGB tunes.