I think they’re fine just to keep you name infront of clients/prospects  and maybe a bit of education depending on what you’re putting in there but I don’t see it being a big revenue producing activity.  I certainly wouldn’t use any newsletter that required postage.  I’d use something that could be emailed and you could probably do something there relatively inexpensive. 

I would say if you are going to write it yourself and personalize it (to your firm), then it's worth the effort (but e-mail is best as Baldy stated).  If you are just going to buy a canned newsletter and mail it out, I think you are wasting your money.