I, personally, didn't even start reading the series until the ending of HBP was long spoiled for me, and I can honestly say that it didn't detract from my enjoyment in the slightest. Then, I read the whole series again, and I already knew the ending for all six books - and they were still good.
Spoilers interest me, and highten my anticipation, while blunting my impatience and desire to have the thing over with already, just so I can find out what, exactly, happens. I can honestly say that I'd never have read the series without constant exposure to spoilers.
People who freak out about spoilers are among the most ridiculous people alive, in my opinion. It's self-indulgent and silly, and really has no logic to it. A good book, movie, or whatever is still just as good if you know what the ending is. If you can't enjoy the rest of the experience just because you know what happens later, you really need to learn to concentrate.