I originally bought my Airhawk for the Africa Twin, but just as I ordered it I decided to change to a BMW GS so my Airhawk has been installed on my BMW 1150GS for the last few years.

So what's it like..... well it's outstanding. In my opinion of course. I'm well aware that no everyone get's on with them, but I think that's because they don't spend the time getting them right.

Firstly you mustn't have too much air in them, just a little more than when you can feel the seat is enough.
Secondly you need the one's with elasticated straps on holding the cushion down and on to the seat. Don't bother with the older versions that don't come with elasticated straps.

They do two types of this cushion the plastic version and the neoprene version. This refers to the material that the air cells are made out of. I presume that neoprene will last significantly longer, however the plastic one has done me fine for 2 years continuous use so far, and it shows absolutely no sign of wear at all! So very good quality.
And here's the best bit you can buy them from Busters Accessories for £50 and then you can apply the busters discount code for 10% off, so effectively free delivery. This is the cheapest I've found them.

So what are the downsides:
When they get wet they really do get wet. You need good water proofs to get your backside from getting damp! But you can just slip it off and use the normal seat. I have done that a number of times.
If you get too much air in the cushion it can feel quite strange, push your nadgers up and just generally feel uncomfortable. Drop the air pressure and then you're fine.

Can't think of much else to say really other than try one.