Retention-less pedals work fine with fixed gear for me. I use flat/wide/light MTB/BMX platforms with sharp little metal spikes---Fyxation Mesa MP, Wellgo MG-1, Wellgo B132. The spikes keep your feet planted, and after a couple of shin grating incidents you learn respect for the spikes. Gear choice (low enough to struggle up hills, high enough to avoid spinout downhill) and brakes prevent foot ejection on downhills. If I have trouble keeping up with the spin downhill, I slow down a bit. I can coast over 40mph downhill on a geared bike with little trouble, just tuck and grin. Rarely hit over 28mph downhill on fixed, close to 150rpm on a 70" gear.

I use an S3X on a Gunnar Cross Hairs commuter, mid/high gear when I'm fresh in the morning, low/mid gear returning at night after The Man has worn me down. There is some noticeable lash, maybe 10-deg of rotation or so, and a click/clunk when you shift. Some can't get used to it, you do lose a little of the unimpeded direct-connected fixed feel, but I'm easygoing and find the tradeoff worthwhile for the convenience of gearing. Not the right hub for skid-/skip-stopping and trackstanding.

My commuter is a kinda heavy build, so it doesn't see longer road rides. I might shift once during the commute, down for the uphill or up for the downhill, I'm definitely not working the gears like I would with a geared bike. I'd like to have a lightweight build for longer rides, I think a lower climbing gear and higher descending gear would extend my fixed ride range. Also thought about an S&S coupled travel frame with underseat/seatstay shifter, allowing all the gearing bits to live on the rear half of the frame.