Publishers mostly rearrange chapters and change one or two insignificant things and sell it as a "new" edition just to force new med students to buy more of their books. I basically stick with used books, even older editions as I dont see the need to invest in them. I use, for instance, a third edition netter which serves me great. A new "fourth edition" just came out in which they basically gave it a nicer looking cover and rearranged the image plates. If you can find an older edition netter in decent condition for half the price dont even bother investing in the new one.
There is one ecxeption though. The new edition of the anatomy BRS is definately worth it. It is a huge change from the last edition in the way it is formated and presents the clinical correlations, plus it has a chapter review at the end of each chapter that goes over the most important points. The last edition basically gave you a dry review with questions at the end of each chapter. As far as I know that is the only "new" BRS worth investing in.