The B3 was the first Studio Projects microphone I experienced. At Musictech College, we have a somewhat vicious policy of limiting students' access to microphones based on their time at the school. 1st and 2nd semester students are not allowed to use our best microphones and, until the B3s appeared, they were not allowed to use any of the school's large element condensers, except for some beat up AT 4033s. When the B3 appeared, at a $159 price point (street, including shock mount), it became difficult to tell who was getting the short end of the microphone selection stick.
We compared the B3 to a variety of "classic" comparable microphones: from both versions of the AKG 414 to Neuman's U87 and the B3 was never found to be a clear loser in any shootout. In some situations, the B3 was preferred to microphones that cost 10-40X as much. I've used my "matched set" of green dot B3s (there are red dot and blue dot B3s) everywhere from classical violin and flute to R&R kick drum. In between, I've used B3s in Blumline pairs on horn sections and a background chorus, X-Y and MS pairs on trap set overheads and acoustic guitars and a small string section, and as solo mics on toms, high hat, acoustic and electric guitar, violin, flute, trumpet, trombone, and voice (male and female). While the B3 wouldn't have been my first choice for some of those recording situations, the mic never embarrassed me. Even in a school environment, several of the recordings made with this microphone impressed professionals enough that they purchased the multi-track reel and took note of the microphone used.
The B3 offers three polar patterns: omni, cardioid, and figure-eight. The directional polar patterns are consistent out to several feet, although the critical distance in most of the studios where I've used my mics doesn't allow for much more than 6' source-to-mic before room reverb mucks up the recording. The B3 also provides a high-pass filter and a 10dB attenuator, but this is a 3-position switch that only allows a user to pick one option of three. That has been a hassle a few times, but it's usually not much of a problem.
The B3's proximity effect isn't pronounced (for a large element cardioid), but it's usable by an experienced vocalist and can add considerable low end when used close to a kick drum or tom or acoustic/electric bass. The high-pass filter accurately compensates for close (~2") proximity without tossing away the bottom end of the natural signal.
Personally, I think the B3 is a steal of a microphone. I've been expecting Studio Projects to raise the price for the past three years. Since they haven't, I suspect they are either manufacturing wizards or still waiting for the market to catch on to this incredible deal in a large element, programmable polarity, condenser microphone.