The only thing I've heard in regards to viruses/hackers is some celebrities have had their account PWs cracked and the hacker posts bogus "tweets." Don't click on links you dont trust. Just use common sense as there are some spammers out there, but nothing out of the ordinary that you wouldn't come across in any other part of the internet.
It can be useful depending on who or what you follow, for example your favorite companies, musicians, or club. None of my friends who have it actively post tweets (who needs it with facebook?).
Twitter's value is the ability to follow. I have only used it to follow people I am interested in hearing from, which is a short list that I have made even shorter. I have used it to follow certain traders and investors, and that has been quite interesting, to see how other people trade. You can only know what they tell you, but I do find some value to this, although I am not quite sure how to use it.
You can have updates sent to your mobile if you choose, but this can get quite annoying. I prefer to check twitter.com except for the few I know who only rarely send out tweets.
How have you used this for SEO? Linking a website and including a sentence in there with keywords? I am very interested, thank you.