I'm not a huge twitter user personally, but I've seen it best used for doing four things.
1) Use it for short info blurbs that aren't worth the company blog. Ex: server downtime, 1000th user announcement, single feature tweaks
2) Use it to get immediate feedback or informal polls from users. Ex: We just released this new feature, what do you think about it?
3) Use it for short, informal customer support. Ex: a user has a very short question, they tweet a question to you and you respond back to them publicly so everybody sees the answer. This is also great when combined with 1
4) Use it for small competitions. Ex: you tweet a trivia question and the first five to ten responders get a prize.
Whenever I put out a new release I announce it on twitter.
The twist here is that I announce it under my personal account (@secretGeek), which has some followers, not under a product-specific account (as some startups do).
If I were to use a product-specific account then I'd not only be trying to get visitors for my product, I'd be trying to get followers for my product's twitter account. The less things you have to do the better.
When twitter is integrated into the overall social media strategy I have seen very powerful results.
An overall social media strategy includes the development of long term value-added relationships with customers and stakeholders of the company.
The most effective use of twitter I have ever seen is on the brand management side. I was with a "twit" (one that twitters, right?) and he was having a nightmare trying to rent a car that he had ordered on an online travel site. He twittered his frustration. Within about two minutes he had a customer service rep emailing him offering to help. Which they did. Which got us the car. Wow. That just earned a customer for life. Good brand management!
Another effective business use of twitter that I have been impressed with is the pushing of last minute deals to "followers and friends." the local chocolate shop that I love pushes their "extra inventory" about an hour before close and offers a coupon to go with it. It reminds me to stop by and get a truffle, and it allows them to close every day having sold out.
In the end twitter -- as all social media -- is about creating a relationship with the customer (and stakeholders) That relationship should be meaningful, and two directional. Making twitter part of -- rather than the end seems to be the best approach.
The most effective thing is sharing links to content that is interesting for our target market. We make sure to keep it balanced so that it's not just our own stuff.
One tactical thing that has worked for us is using twitter as the preferred method for our webinar audience to ask questions of the speaker. We have very popular webinars, so this has grown our twitter reach considerably.