As others have mentioned, email marketing can be very effective, or it can be a complete waste of time. If your emails are spammy, they will not be read, or worse, they will get you in trouble.
At least for anyone doing business in the United States, there's a law called The CAN-SPAM Act (I refer to it as the CAN'T SPAM ACT) which closely regulates unsolicited commercial emails. There can be penalties of up to $16,000 per violation for failure to comply with Can-Spam.
So, if you're considering using email marketing, you should familiarize yourself with CAN-SPAM'S requirements.