fatboy,
Had a look at binexx last week. As you say, not much liquidity. The main business they are going to attract initially will be arbers of course.
That's no bad thing, but my overriding concern when I get into a position is where am I going to get out and sometimes on the p2p exchanges, you can't. Still, if you do make use of binaries, increased competition has to be a good thing.
Imo, binary/fixed odds bets in general, are a reasonably cost effective way of trading volatility. Having said that, you will find that everybody has more or less wised up now on the exchanges and it's getting harder to get a decent volatility based strategy on prior to non farm payrolls etc. I have to say, call IG all you like, but at least they are always there to take the other side of your trade (in theory...
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If you trade price, i.e. direction, then (imo) binaries are a waste of time (unless you use them intelligently as a hedge against another position). It's all too easy to get shaken out of reasonable positions. It's also easy to bloody mindedly stay with with positions that you wish you'd got out of and then find that you can't.
Above all don't labour under the mis-apprehension that the exchanges are full of "punters" who will be easy pickings, or that they are better simply because you will never get an order refused. These days they are frequented by some pretty smart cookies who know every floor based trading trick going. You will get spoofed, dimed, squeezed, etc, many, many times.