I'll admit that I'm sort of agnostic when it comes to Blue's products. I'm not a fan of the Blueberry, the Mouse, the Kiwi, the Dragonfly, or Blue's landmark mic, the Bottle with its stock capsule. However, the OmniMouse sold me. Blue recommends this mic for Decca tree classical recording situations, but I've found it to be useful and interesting on a variety of projects, from vocals to room mic'ing. According to the Blue specs, the B4 capsule is a small element "true pressure omni capsule." According to a conversation I had with one of Blue's engineers, "small" means approximately 3/4" to Blue. Based on my experience, I would have suspected something about that size might be hidden inside the Omni's rotating capsule housing.
For one, the "omni" quality of this mic is mildly directional. There is no noticable bass-boosting proximity effect, but there is some high frequency directional character. That is probably partially due to the large housing ring that surrounds the capsule, focusing some of the 2kHz and above information into what becomes a hyper-cardioid pattern at 16kHz. You can easily hear this effect by rotating the element while recording a moderately broadband signal, like an acoustic guitar. A more omnidirectional omni would produce unnoticable frequency variations under these conditions, but the OmniMouse is not specially omnidirectional.
That's not all bad. If I wanted an instrumentation omni, I'd buy a small capsule unit with a small body. I was looking for a musical tool when I purchased a trio of OmniMice (Mouses?) to stuff into a Decca tree. The biggest surprise was in how beautifully this mic reproduces a near-field vocal. I hadn't planned on using it for that application, but after accidentally trying it on a voice, I've used it often ever since on male and female lead vocals. In several recording sessions, I've received raving responses from vocalists and voice-over artists after they've heard the playback from the OmniMouse. I've also used the OmniMouse in conjunction with a Royer R122 in M/S configuration to pickup a horn section and that was about as realistic a section as I've ever recorded. It just jumped into the mix with depth and width, far more interestingly than with either a U87 or a C414ULS in omni mode.
Once I realized this was a more versatile tool than I'd anticipated, I began to use it on all kinds of acoustic sources; from studio group voices to acoustic guitar to solo flute and classical cello. I have yet to find an application where I've been disappointed by the OmniMouse and I can't recommend this mic too strongly. Now I wonder if I'd like the Bottle with the B4 capsule?