Personally, much as I love my SS road bike, I don’t find it’s a good training device – quite the opposite, since as soon as you get up to speed it doesn’t want to go faster (I’m on 42:16), and generally I put just as much climbing effort in on gears anyway. Gears make me work harder overall. But I’m a bit of a big old masher – I don’t like spinning fast.
The SS is perfect for winter/wet rides, though, and it’s great for forcing the muscles back into shape if you’ve let yourself go a bit. And it’s just nice for just riding along without bothering about gears or having the slight noise of the chain turning jockey wheels.