It's definitely very good for BOAS, quite fantastic really. But it's interesting that they had some trouble with BIFs in some cases, perhaps due to the lag and blackout. With my old fashioned mirrored camera, as long as I can keep the bird in the focus zone it will do quite alright. But I'd like to see more reviews to get a better sense if it's really a camera problem or just something that requires a different technique.
I can see the R5/R6 being absolutely ideal for something like the trip I did to Costa Rica, where you set up your rig at the "bird studio", and blast away when they come in for a quick perch. Having a camera that can grab the eye quickly would be brilliant. Often you only have a couple seconds to shoot before they jump down to the fruit tray (which you normally try to hide from the shot!).