I've put a few rolls through using a Rolleikin - mainly for the novelty value. However, it is a nice set-up for portraits giving you a 75 or 80mm lens on 135 film as Bob said. I've taken some nice shots of my sons using the 35mm adapter. This guy
has done some really nice stuff using a Rolleikin. One disadvantage of using the set-up is that it's VERY difficult to do any pictures in a landscape orientation without using the sportsfinder or a prism.
Here are a couple of my examples. Don't have any portraits scanned at present.
Some of these are Rolleikin + Rolleinar - making the most of those Rollei accessories...