I use a three speed on one of my fixed bikes - my commuter. It means I can set off smoothly from the lights,and change up to the 90" top, to get a good speed up without spinning my legs off. It helps descending too. The lower gear is useful for climbing. Doesn't match my tourer, of course, but it helps with the ups. I've ridden the S3X off road, and taken it cycle camping (London-South Downs).
It's a wee bit different from riding normal fixed, as there is inevitably a bit of 'lash', which feels wrong at first, as if your chain is slack, but it gives you a good window for changing smoothly. Because of this, I don't tend to use backpedal braking, and have a rear brake fitted.
It does make the bike heavier, though that's not an issue for my commuter (I could still leave most MAMILs standing). It certainly helps in hillier areas.