I mostly ride fixed these days but I'm not diehard about it. I just like riding fixed. But I also like riding singlespeed. And there are definite plus points to SS, as I'm always reminded when I find myself spinning downhill at 180rpm...
As for the choice between SS and variable gears... I used to think having lots of gears was a good thing, but these days I tend to favour less choice.* I even converted my Brompton to singlespeed. And my last bike purchase was a singlespeed CX machine. I just love the simplicity of having only one gear, whether it's fixed or free.
Generally, I think I prefer fixed for riding on the road, but off-road it has to be SS. I've tried a bit of light fixed off-roading and I didn't much like it. Singlespeed cyclocross is massive amounts of fun and takes me back to my youth, when I used to spend my weekends racing my BMX round the track my friends and I built in our local woods. I don't think I ever wished for more gears in those days.
d.
*This goes for many other things too, not just cycling - for example, I prefer restaurants with shorter menus. Choice is often an illusion anyway. Who really needs it?