I got a fixed-wheel bike primarily because I was sick of the maintenance of my previous one. I wasn't sure I'd like it but I was curious (and brave!) enough to go for it on the maintenance / low cost / light weight issue alone.
I'm pleased to say that I have spent about as much time over the last few thousand miles maintaining it as I spend on my girlfriend's geared bike every couple of weeks!
Without trying to get all spiritual and "zen", I also find that riding fixed is a lot more enjoyable and freewheel bikes seem somehow slack (or perhaps, flacid

) in comparison. There's just something about the directness of the bike that makes it so satisfying to ride. In this respect a singlespeed "cop-out" nets you the worst of both worlds.
I have also become a much stronger rider over the last year and can now manage both a high cadence and/or a high power output far better than before, while my climbing ability has gone through the roof (ahem!). Last year I managed a 10 mile time trial in under 25 minutes which put me above the 'normal' geared bikes and even a couple of the proper TT bikes. So it's not that slow!
I'm sure I would be able to go faster downhill on a geared bike... but really that's not important to me. I can get up to about 30mph anyway, and it's not like you're putting effort in to spin your legs, the bike is doing it for you

There's no question that it might not be the best choice if you live on Alp d'Huez and you only ride for gentle pleasure. But then, Edinburgh is not so flat... for those who like a bit of zing, fixed might be worth a try!
PS. although this long ramble might make me sound like a keen racer, in fact I've only ever done 2 local time trials, and 99% of my riding is pure utility.