Follow some people you find interesting, then they'll reference others so can start following them too... then on a friday you might see a #followfriday suggestion to follow some others...
Before long you'll be following 300 odd people and have the same again following you back

I've made a couple of good friends who I've actually met now... I keep in touch with old friends, I've also started producing a podcast (
PhotoLegal in case anyone's interested

) and got photography work with some people I know through Twitter.
There's a lot of interesting people - check out the search and the trends lists to see if anything catches your eye. It's only interesting when you're involved in it properly - if you're passive then you'll never get anythingout of it.
If you've got a business or trade that you want to advertise then you can develop things using twitter - either as additional advertising or through contacts on there.
Now that I've got a decent group of people following me I actually use it for questions and advice in preference to Google - especially if it's about a purchase. I've got great advice and have even been lent equipment to try before I buy from people who I've never actually met.
I'm @incapete if you hadn't guessed! Come say hi!