The Bx V2 is the best mastering Eq in the UAD arsenal in my opinion. I'ts just easier to dial in than the precision eq and the stereo expander is the least annoying that i own, which means i probably use it more than i'm supposed to!
Very smooth filters, incredibly handy 'auto-solo' feature for finding the right frequency quickly, a smooth de-esser and mid-side functionality make this a winner.
My only beef (and I am sure it is a Brainworks design thing, not a UA thing) is that some mixes get a little phasey and washy if the mid and side eq curves are radically different or the width is too high (sometimes even 105% will get ill-defined). But that is a minor quibble compared to the beauty of this eq's functionality in other areas.
And don't forget those sweet filters!
In my tests I find the UAD seems to have more non-correlated information than the BX version, even if set flat. Also, bypassing individual bands, I hear each filter of the BX version change against a phase-reversed file, while the UAD version has a constant (yet more different) sound no matter what bands are on or off. This suggests that the BX version is removing individual bands from the audio path when turned off while the UAD version is not. That can be good or bad. The UAD starting point seems less neutral (sonically and in a null test), but individual band changes may be more subtle as a result.
Overall, I find the BX to be "softer" while the UAD seems a bit "sharper" sounding. This also suggests differences in phase response.