I tried the FG thing and just could not wrap my head around it despite a few tries at it. I realized that I LIKE to be able to coast along now and then. So I switched the bike to an SS. It's been like that for about 6 or 7 years now and it's one of my favourite work commuters. Mind you my commute route is pretty flat but I set it up so I could spin along at 25 kph and that gives me just barely enough to grunt my way up the 4 parkade levels to where I park the bike in a private garage.

There's no doubt about it. Without the derailleur the bike is just a little bit but very noticalble amount more efficient. Less IS more.

If your route has ups and downs then I'd have to say logic should prevail and you should stick with gears. But if you have lots of route options that are fairly flat then sure, get one. They are great fun.