I've been riding fixed (48x17) for about three weeks now (350 miles).
I haven't found it as compelling or different as I expected. Yes, one needs to adjust to constant pedaling in the first few times, but not that big a deal as I pretty much pedaled constantly anyway before, except around corners.
But honestly I am a bit frustrated with the gearing, I ride only flats, but lately often with good winds. Somedays I with a tailwind find myself wanting a higher gear pedaling at 130rpm, going 30mph (where I used to go 35mph on my geared bike) - but more frequent is the 28mph range where I am at 125rpm, but wanting to go faster. But I find my starts from green lights in traffic a bit sluggish, not as quick across the intersections as I like, so a higher gear is not an option yet.
Of course this is why a fixed bike is so great, it forces you to both strengthen legs and develop higher cadence. Even after these few weeks I am stronger. But I do often want two gears, one for strong headwinds and fast starts, one for top speed.
I rode the many geared bike today on a whim, and didn't notice any transition issues, ride basically felt the same.
But all this said, I am compelled to ride the fixed, I want to, I regret not riding it today, wish I could for the ride home.
Al