Jules 101
I didn't include CityIndex or Finspreads, or any of the other companies which offer limited binary betting opportunities, because their main businesses are spread-betting. The ones mentioned seem to be heavily weighted towards binary betting.
Not Quite Random
I'd put binarybet and extrabet at the top because, as you say, they are slick and simple, and they suit my betting approach.
BetOnMarkets, and possible Betaexoticbets seem more suited to longer-term betting where we can take into account possibly forecastable world events to predict the behaviour of markets, i.e. how an American attack on Iran within the next six months will affect the $.
Not quite sure about Choiceodds yet. I had some success yesterday, but only - I think - because I was able to replicate the bets I was making with binarybet.
RIWF