I will play it. I am convinced they will change just enough to make the game better without it being totaly different (close to what they do with SW Saga, I would gues) and as I did not buy any class options books or the like for 3.5, why not? Of course, my collection of monster manuals for 3E will not be worth more than my 2nd MMs, but I even use those on occassion.
I like to buy a good RPG book every 1 or 2 month, and during the time it would take to finish running campaigns I would get the new core books and game on.
As for the 3.5 stuff being useless, how many books do you expect them to release at once? I think many things will stay quite useful, I expect the difference to be somewhat like AD&D to AD&D2nd. (But I hope 4th Ed will be a cleaned up version of 3E, not worse than it.)