It's certainly a watershed camera (as others have said), but a classic? I'm not sure about that.
Having said that, I'll also say that it's the first digital camera that:
a) Fit my wallet, and
b) Made me think that the consumer-level digital technology available was as good as or better than film.
Up until last May, I'd always owned and used film cameras. The goodness of the 5D and the convenience of digital drove me over the edge to switch. The high-ISO performance was/is icing on the cake; I'm still amazed by what the camera can do at higher sensitivities that high-ISO film simply cannot.
I guess the implied question here is whether or not you should buy the 5D or wait for its successor. I can predict, for its successor:
1) The image quality will not be so much better that I'll want to upgrade.
2) The ergonomics may improve somewhat, but (for me at least) the ergonomics on the 5D are pretty good already -- I won't need to upgrade for any improvements there.
3) The image capture process may be faster than the current 3 FPS. If this is very important to you, then wait. It's not for me.
4) There may be a better way of dealing with dust (a la the 400D). For me, though, dust hasn't been a problem that I couldn't eradicate with a puffer bulb (yet), but if it's an issue for you, you may want to wait.
In short, unless something changes radically for the better, I'm unlikely to replace the 5D anytime very soon. Then again,though, I kept my 630 for 15 years (and my EOS 3 for one year after that before I upgraded to the 5D), so I may not be the best yardstick by which to predict these sorts of things.
Still, though, I believe I've never owned a nicer piece of photgraphic gear than the 5D, and it'll take a fairly significant leap in camera goodness to wrest mine from my possession.
...and I just don't see that happening.