I have an old Pearl ST8 stereo multi-pattern capacitor mic from the 1970s which uses their rectangular capsules. I like it a lot, and to my ears it has some distinctive characteristics, but I don't know whether these are the product of the capsule shape or some other aspect of its design. It seems to me to have a somewhat more 'focused' sound than many conventional large-diaphragm mics, by which I mean that its directionality in cardioid or fig-8 seems quite sharply defined, with fairly well-behaved off-axis response. The on-axis sound is typically slightly 'hard', with a hint of emphasis on the upper mids. In some ways the sound seems to me closer to a small-diaphragm mic than to something like a U87. It's hard to describe in words, but it feels tight and precise rather than expansive and lush.
I have used it to good effect on acoustic guitar, drum overheads and so on, but I've never felt the urge to try it as a vocal mic.