I think it depends on whether you are using it for a particular purpose. I am a freelance journo and it has really helped me to build a twitter profile and connect with people who also specialise in and around my subject area. It looks good to have a lot of twitter followers too - even if not all of us quite know how we can use them yet!
Everyone I follow seems to tread the appropriate line between sharing and openly trying to promote or flog you stuff. You have to really connect with other people to use it properly and by retweeting anything I have found interesting I have picked up more followers and had other people in turn retweet my posts. I find it helps to be generous with your tweeting and help other people when you can.
Also as someone who works at home on her own it's been great to find myself a little community - rather like I imagine some people enjoy MN (which I am beginning to too). It's also good for generating hits to websites or blogs if that is in your line of work (or indeed hobbies) too.
When I first started off with it I wasn't very active and I would just follow things like Sky and BBC News and it was nice to have a rolling feed of headlines on my mobile every time I had a sec to get on to twitter.
It has also been surprisingly useful in helping sorting out really niggly problems - I recently tweeted venomously about BT messing up my life and was contacted by them immediately, from someone in the UK, who dealt with my problem and had a refund cheque in the post in the hour. Something I had spent weeks previously trying to arrange unsuccessfully via call centres and lengthy phone queues. I travel for a living and it was also great for keeping up to date with the airports when there were problems with the snow.
When once you get into it and develop your own community it can be a lot of fun, but I can easily see how someone who isn't self-employed, isn't promoting anything, sees all the friends they want to on a regular basis and has enough virtual social contact through facebook already wouldn't need to bother with it.
Phew. Was that too much...?