It is both a plus and a minus in the blog world today.
A plus in the sense that it can help you earn a few dollars to help defray the cost of whatever it is you need help in paying for. A minus in the sense that it earns you squat and can turn off your readers.
To clarify, I'm only talking about the regular advertising that you see from AdSense and other reputable companies, that are strategically placed around and about on one's blog. I'm not talking about the other bits and pieces of self-created advertising (some of which is very good) that you put into your blog, in order to drum up business for whatever it is that you do for a living or hobby (of which I am guilty of doing as well).
Those ads I have no issues with, as I find that sometimes it's the easiest way to keep your name out there among the public while you're in between projects. Matter of fact, I find those intriguing enough to actually go and check them out.
As a disclaimer: I was guilty of doing AdSesne for the first four or five months of my blog. But I decided that they were more of a nuisance (at least for the structure of my blog) than anything else. Plus, I wasn't earning anything from them either.
I follow with regularity, 40 blogs and 2 websites. Of those 40 blogs, about half have some form of advertising on them. Of those half, all but two keep their advertising tasteful and pleasing to the eyes. What I mean by that, is that the ads don't deter from the distinct pleasure of reading the blog.
Of the remaining two, one has a slew of them on his, which is due to the fact that he's doing a research piece for an upcoming article about advertising on blogs. For that, I wish him nothing but the best of luck, because he had stated at the outset of his project, that he is looking to expand his horizons (and publishing credits) as a writer. In this case, the ends justifies the means.
The last one, I'm trying really hard to keep following. While it was one of the first blogs that I got hooked into 8 months ago (due to the tips on writing, etc.), it has gotten reformatted at least a half dozen times and had nothing but wall to wall advertising put onto it. While this person has stated from the outset that they're looking to maximize the income from the blog, for the average reader like me, having to wade through gobs of ads splattered throughout the main body of the blog, is fast becoming a major turn-off.
In any event, while I don't mind advertising on blogs (and more power to the people who have them), there has to be a happy medium between having just the right mix/placement and having so many of them that it detracts from the beauty of your blog content.
For the record, I do enjoy tremendously all 40 blogs and 2 websites that I follow on a daily basis, and I will continue to support them the best that I know how.
Additional note: I just want to make it clear to everyone that in no way am I slamming people for having advertising on their blogs. If you want to get the most out of your blog, by all means you should try your very best in doing so. Just because it didn't work for me doesn't mean I don't think it can't work for you. Sometimes a good mixture of professional and personal advertising can add the right touch to a blog.