Guest blogging is useful, in my opinion. Especially when you guest blog for a well known blog (I've done it for readwriteweb.com some time ago and I might do it again).
Guest blogging can bring you exposure, can network you with other bloggers (maybe well known bloggers).
But you have to be selective for whom you do guess blogging...and not do it often otherwise you don't have enough time for your stuff (if you have any).
If I wouldn't take my time to write an article for Techcrunch I wouldn't get visitors and a link back, would I? Not to mention that many other people saw my part of my article...