I've had a few single speeds - riding fixed never appealed. Rather than feeling free I felt confined.

Particularly, towards the end of a year, I like to shake up my riding with something different to keep it fun. Riding a single speed (or racing 'cross) works well for that. I'm rarely without one.

The relative lack of maintenance obviously has benefits for commuting too. If you choose the right gear, even hilly routes aren't bad and you'd be surprised by how fast you can scoot up inclines. Probably my favorite single was a Miyata Triple Cross commuter with mustache bars, fenders, and a rack. I commuted on it and after work, with a full pannier, I'd go do hill repeats a few times per week before riding home. I had my best race season ever after a winter of doing that :-)

Past single speeds:
Schwinn Prelude
IRO (whatever the aluminum one was)
Miyata Triple Cross
Performance branded steel MTB with rigid fork
Surly 'Cross Check
Diamondback something or other - selling the frameset

I'm probably going to get another Cross Check before the Fall, provided I have a job within commuting distance.

Anyway, build one up and have some fun.