I think it may drive a few people away especially from the placement but I've noticed the forum is getting less relevant now and there are alternatives.
I don't see the issue. I barely notice them, they don't add much overhead to the site and I assume they are to help cover the running costs. If people get driven away they probably by a small box at the top of a page I suspect they need what is called 'a sense of proportion'.
I think they're small and discreet. They blend in well and seem to be appropriate to a model railroad community. If they were advertising herbal formulas for enhancement I would definitely object, but these seem to be okay, and maybe useful.
And they help cover the cost of the site, which is a good thing.
I don't mind 'em (they came as a bit of a shock at first) One thing though; auran if your listening: you mind moving the ad so its right aligned instead of off-center aligned? Oh and ditto to what dmdrake said, no animated or picture ones; just keep with the text ones.