They work and some people really find them useful.

Some time ago I've noticed that a few of my friends really enjoy going through newsletter emails and finding out about items that are on sale etc. They wouldn't necessarily look for a newsletter signup form on a website, but when a popup appears something clicks in their brains ("OMG, I really need this!") and they sign up.

I don't like these popups but well, most of the time I'm just not their target.

On the other hand, I'm pretty sure that there were times when I actually got converted by a popup.

When you think of that, if it offers something that's useful to you, clicking on it feels so natural and you don't analyze it too much.