I find them great for looking at data after training and races, but not for referring to during a session.
This is how I tend to use mine (PowerTap), particularly as a lot of my riding is commuting into London, but it's excellent for retrospective analysis, as well as for intervals and what not.
IMO the problem with PowerTaps is that they're heavy, so you either have a heavy wheel to race on (if you road race), or a light race wheel to train on. Or 2 hubs.
If I was buying now I'd get a Power2Max chainset, more versatile than a hub system and cheaper than Vector pedals, Quarq or SRM. In fact I'm planning to sell my PowerTap wheel to change, on the very rare occasions I use my 'normal' rear wheel (which is only a Race X Lite) the bike just feels so much nicer to ride, but I don't like not having the data.
You won't get any feedback on Vectors as they're still not out, and delayed until the summer currently. Doubt you'll see them before 2013 myself. They look good, but they are expensive.
a number I find invaluable on the road - have the others caught up and display that now?
Yep, they'll all do that.