I already have a 6th ed, and I'm planning on sticking it out, but I'm only doing so because 1. the 6th ed was FREE, and 2. my school has the 7th edition available online for all students. The online is kinda slow and annoying to navigate, but it's there. So far the content is basically identical, but I've only been using the book for a few weeks (first 10 chapters or so). What IS different is that the 7th ed has clearer/longer explanations, and it has more cartoon-style diagrams that summarize the info in the text (great if you're a visual learner, which I'm not).
My only real complaint at this point is that my professors often give page numbers in the lecture notes, and they are COMPLETELY different.