I've had good and bad experiences with the combination. I've been drinking coffee on the daily for about seven years, so I'm kind of always on caffeine if you will, but I've only consumed Kava and Coffee simultaneously about three times. First time was a small cup of coffee with ~75mg kavalactones in me, about 40 minutes after consumption. Felt real nice, just sort of mild happy energy. Recently I consumed about twice as much, and about 45 minutes after consumption, did a double shot of espresso - bad idea, tweaked out hard. But there was another time that I consumed some smaller amount of regular coffee while under the influence of Kava and felt just fine. Something something moderation. I certainly don't plan on doing espresso with Kava again, but regular coffee with Kava will likely happen again in the future.
Drbinxy, sorry to hear your experience was so bad - can't say I've ever heard of what you took in combination with it but I'm assuming they're a different deal than Caffeine.