I have a little Penti I and always carry it with me when other cameras would be to big, heavy or expensive.
It's rather primitive and needs film in SL-canisters (very hard to find and you have to load them yourself from bulk rolls), but other than that it's pretty nice. The lens is a 3 element Domiplan 30mm/3,5 and the shutter goes from B to 1/125. But don't be fooled by these specs... Stopped down to f/8, it can produce pictures that can be enlarged to 8x12" and still look decently sharp. Of course you have to deal with massive grain, but that's one of the reasons, I love the smaller formats. With the slow shutter, the most used film is Polypan F at 50ASA.
I'd love to have a Pen F, but that's just a dream for the day I have too much money to spare...
I also built a half-frame 120 pinhole camera once... it took 24 3x6cm panoramic pictures on one film. That's one pretty cool format and I really wonder, why none of the "real camera makers" have ever thought of that.