That seems to vary between subreddits. I've encountered a few quite successful ads (with comments from regular visitors to the sub further supporting the product!). /r/sysadmin comes to mind, but I'm sure there are others.

XX (or is it Dos Equis) and Oldspice would beg to differ, and they didn't even advertise there. I've found reddit to be rather accepting of advertising as long as it is done transparently. IAMAs by celebrities every time they star in a new movie aren't treated with hostility - just put the fact that you're in a new film in the submission text.

There have been many HN posts and comments about successful paid ads in specialized communities.

Well, there you go. When you know the audience, just adjust your advertising to fit. Not that hard. Not every product should be advertised on reddit tho.