I highly recommend reading the excellent documentation, the Pro Spring Integration book, reviewing the samples from the git repo, and Josh Long's in-depth article from InfoQ.
However, when I was first introduced to SI a few years ago, I had a few questions who's answers weren't immediately available from the perspective of someone new to the integration space. I've tried to write a blog post from that angle to try and recapture some of those questions and hopefully give good answers.
Questions like "what is a mediation framework", "why use SI when I can just Spring directly" and "how does SI relate to an ESB?" at the time (for me) were not immediately answered by reviewing the documentation. Hopefully my blog post answers those questions for those new to SI and coming to the same questions.