I have to say that i hate them, they interupt my very fast skim reading of the posts between the OP, and the last post. And that i will never ever ever click on one.
My main problem with Google Ads is that the design and look of them isn't good. They cheapen the look of the site.
The fact that anyone can get google ads takes some of the value away from the site. With paid for banner and tower ads, people see them and know that people have paid a sum of money for that space, thus adding to the perceived value of the site.
I don't think you'd see google ads on my site.
If the ads were relevant and in defined places then no problem but there is no consistency of content. However, there is likely to be a bedding in period for the ad spaces to provide relevant options.
If the adverts were displayed to users who were not logged in this might discourage signups unless you had a 'sign up to remove these ads'
The forum needs to be financially self-sufficient as hosting and bandwith costs need to be met, and without membership fees then adverts achieve this.
There are other ways of improving revenues in a controlled way and I do appreciate that if Google is noticing the forum and wants to 'ensure you are placed highly in search results' then through a partnership with Google ads dominating slots then there may be limited options available to you.
Just my tuppence worth of opinion on this matter...