Thanks for the thoughts guys. I don't want to drop £75 on the thing and hate it, which is why I'm seeking opinions from people who've used them.
The main scenario I have in mind is out shooting motorsports. I use one body (typically the 5d) with the 400mm on a monopod, with the 70-200 attached to the 1d. The idea is to have the 5d's strap round my neck, so that I can simply let that rig go, whilst pulling the 1d/70-200 out of the holster to shoot the cars as they come in closer.
Because (and this addresses Scaff's point) I'm using long lenses, a satchel type bag isn't really practical because it would have to be too big to take the out-of-use camera. Also, the switch time between bodies really needs to be sub 3 seconds, so switching using a bag will take too long. Finally, I don't really want to take a bag, because a bag that's big enough to take the 5d + 400mm is too heavy & restrictive. I tend to want to leave the bag in the car.
If I'm using two bodies with short lenses, (e.g. 24-70, 16-35, 135) I'll take the Lowepro Toploader 75, which can take a body with short lens plus another lens underneath, then the second body goes round my neck. I'd potentially use the holster in this situation sometimes, but it's mainly for being outside with the big guns.