Well, there are good and bad experiences with it, if you ask the blogging masses...
If you're new to the whole "I'm gonna use ADSENSE and blog to make MILLIONS! YAAAY!" I have news for you. If you don't do your homework, you're not going to make any money.
And if you are too good, and make too much too soon, google will END YOU, like they did me (more on that below).
Here is an awesome infographic I found from Wordstream:
If you blog about everything, your ads are going to be about everything. Google has tools that let you focus your ad material on more relevant topics, but be careful: If they think you're playing their system, they can shut you off without notice.
I started blogging in January. By the end of February, I had enough traffic to net me $10-20 a day via adsense. About 1.5% of my viewers were clicking ads, and I was making roughly $0.78 per click. It added up, and my first check from Google was almost $200. They eventually cut me off (without reason, just "we decided to end your account") but it was fun while it lasted.
Most other advertising sites don't have the same monetary yield as google, so focus on adsense if you can. Blogher's ads look like they may be decent, but I'm not going to go down that pathway again. (and I hear Blogher is very VERY picky about who they let in...)
For those of you with adsense success stories, feel free to share them by commenting below. I'm sure newer people to the game would appreciate it...
...oh, and have I mentioned you can advertise on my blog? You know, just in case I hadn't already...