I first learned photography on a Yashica-Mat TLR in the sixties in school. I have always loved twin-lens photography since. I primarily photograph with Leica reflexes, but the TLR occupies a definite place for me.
For the beginning darkroom worker, medium format is so much easier to work with than 35mm! Every little speck of dust isn't enlarged so much--it's much more forgiving of darkroom errors.
The square format is ideal for these cameras because it doesn't require turning the camera in a specific orientation. Besides, one can easily crop in the darkroom with the large negative.
The twin-lens configuration, where the phootgrapher looks down into the groundglass, can be ideal for portrait photography, where the subject doesn't get the sensation of being 'aimed at'. This is what I still prefer these camera for--people. Most TLR's use a taking lens of around 75-80mm--a little short for a classic 'portait' lens, but close. Simply concentrate your subject in the central part of the groundglass, and crop in the darkroom if desired. For me, I prefer using the whole frame. Again, there is the flexibility of the square format. I generally have an idea of what I want the photo to accomplish before I take it, so there's no great deliberation afterward.
The Rollei TLR's are all excellent, at least those made within the past 50 years. You can't go wrong with any of them. They are beautifully constructed and extremely durable if maintained. They are as quiet as an M6 Leica. They are often lighter today than some of the 35mm SLR's, and some are smaller also!
Limitations? Only one focal length. However, take a look at the classic photos with the Rollei TLR! Not suitable for every photo situation, but then, no camera is. A single focal length is a great teacher of camera technique and composition. From time to time when I feel I'm being confused with my 12 Leica lenses, I pull out a Rollei for a month and recharge my batteries.
Buy any good Rollei TLR; it's an investment and a camera that can stay with you for life--and then some!