Twitter is EFFECTIVE for your marketing your business
Twitter is a highly effective method for marketing your business online, in that it allows you to have instant contact with a highly targeted niche. Most businesses make the mistake of sending out Sales Tweets -- "See the coolest thing I'm selling you now" -- but that's not really an effective strategy.
It's much better to use the 80/20 rule -- 80% of your tweets about interesting and fun facts, odd news items, questions of the day ("What's for lunch today?"), etc. Get involved in conversations and get to know people. It's not overnight, but it definitely works.
Plus, it's kinda fun. :-) You really end up following some very interesting people.