Yes, I've thought about it too, but like Rafe, I think it would be labor intensive. And then too, I'm thinking I'd run out of things to talk about. I do a blog on writing because it is not so limiting as just talking about my book, and even then, I decided to share it with 2 other authors just so we'd all bounce topics off one another.
I try to publish mine monthly, but as it has been noted, it is a lot of work, and you are the only one with interest in it as a general rule. I would love to get others to submit stuff for mine, as it is a mystery focused newsletter. Maybe in time!
Like most promotion efforts (such as joining Crimespace and contributing to Forum discussions ;-) ), it's very hard to quantify the payoff for an email newsletter. I think it does pay off at least some, because I've had recipients tell me they've bought and read my book. Also, I always have a signup list for the newsletter at my signings, so if someone doesn't want to or can't afford to buy the book then (it's a hardcover), they can still enter my contest for free mystery books and get on the newsletter list. Then, they may decide to buy the book later after receiving a newsletter.
I have about 1800 subscribers and used the Zinester service for quite awhile to maintain my subscriber database and do the mailings, but then they got flakey recently and I switched to Vertical Response. A few other authors I know use them, too, and we've all been happy with the service. I no longer send out a newsletter every month, but I try to do one every 2-3 months when I have interesting news to report.