The benefits begin immediately (priority seating, 10%/appetizer, etc.), but the $10 coupon comes in the mail later. When I called RFC corporate in Nov, they told me it would be SIX to EIGHT WEEKS before the coupon arrived. Since we would only use the coupon on vacation, and we were well within six weeks of our DL vacation, we opted not to join the Safari Club this time. We had a 5 pm reservation on a Monday in December and had about a five minute wait without any "priority."
We may reconsider if we decide we like RFC and want to use it on our upcoming WDW trips (the jury is still out on if we like it that much). The $15 joining fee is for a lifetime of benefits, so if you believe you will use it in the future, it might be worth it . . . but not for a once-in-a-lifetime thing if you won't have the $10 coupon by the time your vacation arrives.
JMO, of course. YMMV.