Been riding fixies since about 1973, mostly in the winter when I was racing, but for the past few years all of the time (except when I'm on the tandem). I did a randonneur series in 2009 on a fixie, and have done centuries and stuff. I don't find riding a 10-sp carbon fiber road bike on a century very challenging at all, but riding the fixie puts the challenge back into it, especially up the hills. Also nice to blow past guys on fancy road bikes, plus there's no shame in getting dropped when you're on a fixie! A real win-win.

I usually ride 42x16, a good compromise for moderate rolling and for up and down hills. I've maxed out at about 67 kmh on a descent in 42x16. Not recommended, though, unless your chainline is perfectly straight!

Fixies are a big advantage in winter, as you can feel if the road is the least bit icy when you use the rear brake (I always use two brakes, although I like Sheldon Brown's recommendation to use only the front when it's dry). It's also nice at night as you don't have to worry about what gear you're in when it's pitch black (a problem with the tandem - I look back at the rear cluster at night and it's too dark to see it!).

Here's a photo of my Rodriguez "Shiftless" street track bike (ordered in 2009 with couplers) at the top of Hurricane Ridge, a long climb just south of Port Angeles, WA:

I had the double-sided hub on the back. I used the fixed side going up, then reversed the wheel at the top so I could freewheel down on a single-speed. I'm not stupid! Well, not THAT stupid!

L.