I was road cycling for 30 years then bought my first single speed bike, mostly out of curiosity. It instantly became my favorite bike. What surprised me was my average speed didn't really decrease and hills/headwinds didn't hurt like I thought they would. It does make you a stronger rider no doubt. After awhile I wondered why I even had bikes with multiple gears. Very simple to clean and maintain and you don't have to deal with tuning. If I lived in the mountains it would be different.
The bad part for me is the limited selection because I want road geometry and bottle mounts. Not track geometry, no bottle mounts or a lack of brake mounts. But being a vintage bike kind of guy the Wabi Special was perfect.
