I enjoy my Salsa Stormchaser single speed (not fixed gear) for the simplicity. It's my winter beater as well as rain bike so I don't have to worry about maintaining as many moving parts.
I also adore the climbing, while more than 10-11% grades become a struggle that I cannot sustain for long, on more moderate uphills nothing can outclimb a singlespeed due to the efficiency. I appreciate how with power the bike just lurches forward with authority, it's so responsive. You see that the chain is as short as possible and the number of teeth on all the gears are minimized, it's all about efficiency and lightness.
Singlespeed is also a great neighborhood, urban and bike path solution. When riding along with slower riders, the fam, or in an unpredictable environment its nice having a bike that you just pedal away and don't worry about having to make a sudden stop in traffic finding yourself flatfooted. You have a ceiling speed past which you cannot go so you're not tempted to speed off as much (though to a certain extent you can). Also very nice to have a bike that takes less of your attention as you focus on your family, the kids, your friends, the conversation and your surroundings. Above all the single speed allows a gear that is just short enough to provide strong acceleration but tall enough to sustain speed for blasting across intersections quickly, both the crossings bike paths have with roads, as well as intersections in the city.