I couldn't imagine introducing a person to the series anywhere else.
I saw the series three times, before I got the change to see the pilot, since WB goofballs thought including it in the entire DVD boxset was not nessecary.
My girlfriend saw the entire series without watching the Gathering. A few months after, we finally got a change to watch it, and really, as fun as it was..... it was pretty bad. The storypoints are picked up in the series so well, that's it's really not nessecary to view it first.