Actual advantages of SS mountain bikes, compared to gered:
- 2.5 to 7% higher transmission efficiency, compared to even the most efficient derailleur systems.
- No chain slap.
- Lighter than a geared bike (no deraileurs, no shifters, no cassette, no additional chainrings, shorter chain.
- No junk to remove from the derailleurs at the end of the day (or, sometimes, during).
I also like the fact that I don't have to think about shifting. You don't even realize how much brain-real-estate that takes, until you get back to a geared bike. So, at least for me, singlespeed bikes have been even more relaxing than geared.