I've been using Tweet Library as a supplemental Twitter client since it was first released. It has matured with every release and 2.0 makes it a universal app. If you want a good review, visit Macstories.net.
I wrote a review of Tweet Library about 6 months ago but never posted it. The take home point was that TweetLibrary is a capable app that could be my only Twitter app but it's a bit too slow. 2.0 appears to speed the app up but it is still much slower than recent additions to the field of Twitter clients.
That said, it is the best Twitter archive and research tool available anywhere. The app downloads and saves Tweets (not just links or references) and can export and share them in ways no other app can. If I'm doing research on Twitter then I do it through Tweet Library. If I'm just catching up or wasting time, I do it through TweetBot. Tweet Library is a serious tool.