I've read this whole thread and its entertaining repsonses. I'll offer you my take based on personal experience to seriously answer your question. I say ride an fg bike for fitness and fun. Assuming you're a normal rider without any physical caveats, a road geometry bike in fg will afford you the ergos in the cockpit that could enhance your riding experience/fitness results.

I have one multi-geared road bike, two multi-geard mountian bikes, one drop-tube flat-bar ss road bike, and track-ready-ish bike in fg with a more traditional track-ready geometry and ergos. ALL of my bikes give me a workout when I ride...or not...I always get what I put into it.....or not...its simple logic for me.

However...out of all the bikes at my disposal, I have found that riding my Soho-S in fg and now my Madison in fg has given me the best workouts while out riding. For me, 1-2hrs of trying to maintain 80~90rpms between stoplights and stop signs in fg is harder....but...I find it more fun and satisfying than riding the same routes on my roadbike or mountain bikes. Why? I think its because I'm not allowed to coast, I'm constantly working for it, and I have to ride up hills or walk and destroy my cleats. I always choose ride. Its a challenge at times for sure, but its a worthwhile challenge for me. Let me also say don't be stupid: choose your route with respect toward your gearing in fixed...you want to challenge yourself, not damage your knees.

There's something very satisfying for me about spooling up to 30mph (its all I can do for now...hell maybe ever, but I think I might have more later ) from a stoplight/stopsign with my heart and lungs feeling like they're about to come out of my mouth as I try to hold that speed for as many seconds/yards as I can...then backing off the pressure but knowing I can never coast and rest my legs like I do on a fw bike....my legs are always working for those 1-2 hr rides. I've been doing this since April 2010 and I've lost about 20lbs so far and have gotten stronger.

Yeah...I say ride in fg...use a sensible gear for your currrent fitness level...spin in the 80s minimum...ride a sensible route...you don't have to go out long...you just have to go out strong for the time your out riding...1hr in fg will feel like 2hr in fw because you can't coast, you can't rest your legs, and you're constantly working.

I think you'll like the results if you're not too lazy Oh yeah...get a good saddle 'cuz you'll be on it lots unless you like to stand from time to time to mash or stretch out a bit while riding. I hope this perspective has helped you decide. Good Luck and Good Riding!!!