Instagram 2.0 is available to download via iTunes app store. Those that already have previous version, simply run an update through app store application. This is a must update for all users since it has quite a significant changes in term of performances, UIs as well as new filters (4 more actually).
IMHO, Instagram is one of the leading photo-micro-blogging application on iOS at the moment. As people usually say, the pictures worth 1000 words. So, by mathematically, it beats the hell out of twitter by almost 10 times :-P Joke aside, I would say that instagram has built another successful micro-blogging community – if you niche yourself well enough, there is always a room in micro-blogging: just don’t try to make another clone of twitter, blogger or even tumblr, that’s all.