A Twin Lens Reflex is a great way to try Medium Format
photography, and see if you like it.
Since these cameras used to be really popular, now you can find reasonably good models used and in good conditions for less than what a crappy SLR
would cost new .
Of course, they are interesting only if you would like a larger negative (or slide) than what you can ever get with a 135 camera.
One word of warning, though: don't ever shoot Velvia just to give it a try. Because, you know what will happen ? You will drop that 6x6 Velvia on the lightbox, and then you will start poring over it with a loupe. Then you will realize how incredibly big that slide is. You will get lost into the detail. Tonality will reveal itself to you.
You will stop respecting your 135 SLR. After a while, you will start saying: "after all $5000 dollars for a beginner Hasselblad system is not that expensive".
You are WARNED.