Yeah most people (including people from this forum) seem to hate it. I really don't know why.
It has a sonic footprint which it stamps on everything and I find it de-natures the sound.
It also made it difficult to balance BBE'd sounds and non-BBE'd sounds in the mix.
I googled it a bit recently and found a datasheet for the chip with a block diagram, and looking at that I understood why.
The BBE splits the signal into high, mid and low bands (using a state-variable filter circuit). It uses a transient detector to vary the gain on the high band. The lo-contour control adjusts the level of the low band. The three bands are then summed again.
The problem comes from the fact that when you sum the output of HP and LP filters, changing the gain of one band:
A) it creates a peak one side and a dip the other side of the crossover frequency
B) it changes the crossover frequency
By dynamically adjusting the level of the high band the BBE creates a peak and dip of constantly-changing frequencies and magnitude in the output.
Since an instrument's timbre depends on the balance of frequencies, this explains the denaturing.
And the relative balances of frequencies in the mix between BBE'd and non-BBE'd sounds get screwed with too, because the extent of masking in the affected frequencies is constantly changing.
It creates a sort of transparency, more detail in the highs, but some loss of power.
So you have to be picky about what you use it on. For metal rhythm guitars I really wouldn't.