Aright table top nerds, here's how I break it down.
I have played version 3.5 D&D for about six years now, fairly familiar with the system, I can make all sorts of builds and my group(s) and I can come up with mechanics for most issues we encounter.
Recently, some players in my group started using/buying Pathfinder. The words "Three point seven-five" were muttered constantly and once two members of the group purchased books and the D.M. downloaded the pdf's we actually played a couple campaigns. I have since broken down and purchased the Core and the APG and done massive builds due to a high power one shot we had two months to prep for.
If you can, or want to, buy Pathfinder. I wont lie, its shiny, new and sexy, BUT, it does address a lot of mechanic/customizable options and makes the super-customizable 3.5 into a massive awesomeness.
If money is tight or you have ALL the books for 3.5 already, no pressure to switch.
However, if one person buys the Pathfinder books, you are SCREWED if a second person buys the books.
3.5 is the original, and has "the real system" but Pathfinder was originally intended to be a massive supplement to 3.5, or at least that was the impression I got from reading some of opening notes, and it turned into this massive project.
Fun fact, Pathfinder taught me that Vash The Stampede
exist in D&D, eh?