Apparently, email marketing offers great return on investment compared to other marketing efforts. I believe it! (Presumably that's even if you're not using tools like CANDDi which get other benefits from email campaigns, like identifying web visitors.)

Of course it's important to be sending your campaigns to relevant people, at decent intervals, with decent content, but as long as you use some common sense you can get great results. With careful research and targeting, you can get excellent results! 

From everything I've seen of clients' marketing efforts, I do believe email newsletters/marketing are good things to be doing, and doing often.