I use it a lot.
I would say it's something you need to put a fair bit of effort into, and you need a strategy.
I tend to tweet or retweet items that are of interest to my target market (small businesses, preferably in my locality) and also tweet marketing information and tips. About one in twenty tweets - or less - I'll "plug" my business, or link to it.
I follow businesses that may be interested in my services. Lots of companies have an automatic follow feature, so they follow you back. I also make sure I follow my clients, and people I've met at networking events.
It's all about engaging with people too, so not just broadcasting, but listening and having conversations. I try to limit my time on it to half an hour a day.
It's quite hard work, and you need to put in some effort to make it work. But I have had enquiries through twitter, and I have found new suppliers, so it's been good for me.
I'm @ClaireKerr and @Marketingin1Day if you want to look me up.