How effective is marketing on Twitter?

All I keep hearing about is Twitter this and Twitter that. I have a couple of niche websites that I make money from affiliate products on as well as two blogs, and I have not set up a Twitter account yet. I just want to make sure that I am not wasting my time with it. I am busy enough as it is and if marketing on Twitter is not going to make me money, then I'm not going to bother. So, has anybody seen a real increase in revenue from marketing on Twitter?