I don't have much experience as an advertiser. I've used a couple of free trials with Facebook and Google, but I feel most comfortable advertising on Project Wonderful. Maybe it's partly because my website is a webcomic, and Project Wonderful focuses on webcomics. But I wish Project Wonderful would get a lot bigger, and lots of people would use it for all kinds of things, because there are a lot of things I love about it. Mostly as an advertiser, because for one thing, it's under-valued and I can get really good deals. But also as a publisher, because in both cases, I feel like I have a lot of control.
I always knew Project Wonderful was cool, but I didn't realize quite how cool it was until I started using AdWords and suddenly advertising seemed so complicated. Advertising with Project Wonderful is so easy. It takes a moment to sign up - you just have to upload your ads and you're ready to go. You don't even have to have money in your account before you start advertising - there are always several empty spots where you can start bidding for free on low-traffic sites. I have even occasionally found free spots on high traffic sites and gotten unexpected traffic boosts without paying a cent!
But if you are going to spend money, you can still get really good deals, partly because the website is under-valued, and partly because you have complete control over where you advertise. That's the main thing I love about Project Wonderful. Yes, it's always possible to get a bad deal on advertising, and you don't set your own cost per click, so you have to pay attention to where you're advertising and how well your ads are doing. But the wonderful thing about Project Wonderful is that that is really easy to do! You can choose your own ad spots, track their performance by the minute, and adjust or cancel your bid accordingly. I spent some time experimenting with only a few cents at a time, and found some really good advertising spots that way. If you want to advertise on a bigger and more automatic scale, you can use campaigns to set your bids for you, but you can still monitor all of your campaign bids and cancel or block the sites that aren't performing well.
The main drawback to advertising with Project Wonderful, especially if your website isn't a webcomic, is that not enough people use it so you can't reach a very big pool of people, and a lot of the sites that do publish their ads have very low traffic. But I still think it's a valuable resource for advertisers that not enough people take advantage of, and the more people who use it, the bigger the advertising pool will become. So if you have anything to advertise, just sign up and try out for free.