I started my singlespeed fun with this bike, which used to be an old Peugeot mixte frame geared bike. It was built up as a 42x17 SS using modern components. The LBS that built and sold the bike (Edible Pedal here in Sacramento) even made their own decals to mimic a real manufacturer's. They had the frame professionally powder coated yellow.
My riding partner has a 2009 Specialized Langster Las Vegas edition. I found one my size on Craigslist, so I snapped it up. It is faster than my geared road bike, and it's my ride of choice even on long rides of 60 miles or more. It climbs great, so I use it on rides as far up as Rocklin. Any further up in the foothills and I use my geared road bike. My Langster has a 42x16 gear. My riding partner is 48x15 fixed on one side, 48x17 fixed on the other. Here's both of our bikes. Mine is now clipess though (this is an older pic). Mine is on the left.
SS riding has made me quite a bit stronger. I even do hill repeats on my Langster.