I'm not a fan of Twitter and I do not use it. However, that's my personal opinion as applies to my situation. Many people find Twitter useful for promoting and discussing any number of issues. Many businesses find it a valuable addition to their marketing plan. I agree with Post #4 that Twitter's a tool; its value lies in how it's used. I find that many I converse with are wrapped up in Twitter to the extent that it seems it takes these individuals away from productive work they could be accomplishing. I see this in the freelance writing field - time spent promoting via Tweets, and constantly checking Twitter could be time spent creating a quality feature article, short story, doing some 'paying' business writing and such. We only have so much time to do the things we must get done and that's why I personally find Twitter a time waster.
In general, no technology is either good or bad; Twitter is just a tool. Like most tools, Twitter's impact depends entirely on how it is used. Social connectedness has both positive and negative aspects on people's lives, and only time will tell which aspect is stronger.
As far as Twitter being a waste of time, that too is up to the user. Several studies have shown that the instantaneous nature of electronic communication makes it psychologically addictive to some users. If you are using Twitter so much that it is taking up time that you meant to use for other things, then that is a problem, but the fault lies in the user, not in the medium.
Social media such as Twitter can have positive impact on society. The dissemination of information occurs faster and reaches farther than it ever has in the past. Twitter adds an interesting aspect in that it limits the amount of rambling that can take place at any one time. That being said, the thing that makes it good "the nature and quality of dissemination" leaves little chance for quality control or analysing the nature and veracity of the information being disseminated.
I think that Twitter has changed the face of politics, entertainment, and news. I will admit that when I first heard about it, I thought it would be a passing fad. For better or worse, I think Twitter is meaningful. Yes, it can be a waste of time, but there is often a purpose and benefit to it. Whether or not it is good or bad depends on whether you agree with how it is being used. We cannot paint all tweets with the same brush. Twitter can be banal, but it can also be revolutionary.