Over the last two years I tried the Hogue Holdall Jr, then the Butler Creek slip-on, finally settled on bicycle inner tube for my QA. Hogue's material is comfortable, but I prefer not to have the palm swell and finger grooves.
I'm with airfix. the inner tube is cheap, very thin, rubberry feeling, last great and after the first one their easy to do again.
I bought 1 tube and used it for my P99, my G17, my Kahr PM9, my AR15 handgrips and did a friends Glock and Ruger. And still had enough to cut in 1/4" strips for the Great English Mountain neighborhood rubber band war!!! Still have alot of Mommys mad at me for that one,hahahaha
The Hogue and other grips like it are very bad for the polymer frames. They are so stiff and tight that they can collapse the grip causing it to become slightly concaved. This can then interfere with smooth mag insertion and removal. I had this problem occur with one of my Glocks. The inner tubes won't do this.
At the gun shows, I sell a grip very similar to that of an inner tube. Like Airfix, I also do not care for the palm swell and finger grooves.
One other very good solution to consider is to use tennis racket handle wrapping. It's easy to apply and you can adjust the grip thickness by applying more than one layer.
Walther has something called "dermagrip". It's supposed to slip over the grip and I think heat shrink to it. From the picture I've seen it looks like either bumpy rubber or sandpaper type material. Similar to skateboard tape.
Regarding the derma grip, I checked with Earl's over a year ago and was told the material is like sand paper abrasive type and does not have much rubbery feeling; more or less like the decal grip in nature. Haven't seen one in person yet, I can't really tell though.
Mine gives a very firm grip (it's heat-shrunk onto the frame). It is rough, but that's why it grips. No problem on my hands.
Regarding where to get one, have a look at the front of my dustcover - the chunks chiselled out of my frame are from a scope mount Earl sold me and will not take back. You can find the whole story with a search.
I have dermagrip on my P99. Its not that difficult to put it, but its not easy either. This grip gives a sort of strange but solid feeling. My wife hates it probably due to the recoil of the 99 plus the "abrasive" feeling of the grip. Overall I like it
I've put a Hogue Handall slip on grip on my P99 and it feels great. I'm going to use a hobby/exacto knife to trim away the excess parts of the grip that hang off at the botton as well as trim away the grip at the bottom that covers the Walther logo. We can't very well go and cover that up! The grip is loose there from the rest of it anyway since the Walther logo near the magazine plate is sunken into the frame a bit. I'll post a picture when I'm done tonight.
If you'd like to see my P99 after today's fun (haven't cleaned it yet) with the hogue grip, you can click here. The gold colored checkering is from the screen on my window (the pictures were taken on my windowsill):