The Sturmey 8-speed is nowhere near as good as the Alfine, or even the Nexus 8. Chris Juden of the CTC tested it and didn't like it; the big problem is that direct drive is 1st gear and everything is geared up from that. This means you end up running a ring and sprocket combination like 28 x 20 for climbing, and suffer inefficiency in all the most frequently-used gears.
Back on topic, the gear ratios of the S3X are 62.5/75/100 i.e. direct drive is top. This isn't ideal either (you want your cruising gear to be direct drive) but apparently it's not possible to make a fixed 3-speed that works otherwise. The old ASC was the same basic setup, although with closer ratios. On the other hand, the power losses in a mere 3-speed hub - assuming only one planetary geartrain in use at a time - are minimal and on a par with a really good clean derailleur system.
I was thinking of running 48 x 15, which would give gears of about 53"/63"/85" with 23c tyres. I'd normally ride 48 x 18 fixed (about 70"), but that's just to get me down the hills without blowing the front tyre off the rim with brake heat. 63" would be much nicer on the flat. The hub could be left on the bike all the time; 85" is great for TTs and it would be as efficient as a normal fixed gear.