I use both oak gall and iron gall ink. I prefer iron gall, as it darkens more quickly and has an odd smell which I've come to enjoy. The brand I use is HAXX--which is what Cornelissen's sell.
However, I noticed that despite being cleaned quickly and religiously, my nibs were started to rust. Closer inspection found that this was because the bronze/brown coating (I use Mitchell nibs) had gone, and they were now just exposed steel. I though I was just using the cleaning toothbrush so hard, but experimentation has now proven that the iron gall ink is so caustic that it removes the finish from the nibs. So I set about using the oak gall, and it does it to a much lesser extent.
Using a de-finished nib leads to much less clean lines.
So essentially I can only use a nib once if I want to use the iron gall. They're only 55p from London Graphic Centre (as opposed to 85p from Cornelissen's--ID folk take note), so it's not a big deal, but still, a minor aggravation.
The causticity is what makes the iron gall work so well on perg--both it and oak gall literally eat into the surface. So what makes me love the ink is what kills my equipment.
I would be interested to see if other brands do the same thing. Does anyone else use non-HAXX iron gall?