The Bach 3 is a great option for someone looking for a darker sound, who spends most of their time playing within a normal orchestral range. If you're already on a Bach 3c or similar but find it too bright or cutting, this might well solve those problems thanks to the deeper cup. For me it's perfect when I'm sat down the line in an orchestra, with plenty of sound and weight to support the 1st Trumpet. I also find it an ideal mouthpiece for rotary trumpet work.
Pros:
Orchestral section playing
Big, dark sound
Cons:
Extreme upper register (!)
Slightly less responsive than the smaller cup 3c/3b