People hate popups. So much so that WebCrawler attributes a recent growth spurt to their removal from the WebCrawler site. This doesn’t surprise me – I’d definitely avoid a search site that hammered me with popups.
That said, I think a distinction should be made from mass audience sites like WebCrawler and the typical B2B and B2C sites. Without exception, every client I’ve had has boosted newlsetter subscription rates by using popups. And when they remove the popups, subscription rates go down.
Of course, they can still be annoying, so if you use popups, it’s a good idea to only launch them on the first visit to the site. Cookies are good for this kind of thing.