I have been experimenting with Kava root (Kava Waka), and have been preparing the drink in the traditional way (info avail on google). I have three decades of meditation experience and am trained to know the differences in particular states, i.e., clarity, nonthought and bliss. My specific path is Vajrayana and the practice of prana, nadi and bindu. And I have a good sense of what it is like for channels to respond to a practice or substance. I give this background to help make clear what I'm doing.
I have mixed a concentrated dose of one cup water and roughly four tablespoons of Kava Waka, and I've drunk about three of these prior to meditation practice. My experience has surprised me. It seems that Kava Waka does a very good job of relaxing the channels in the following way. It calms the nerves. Also, it seems to lower the blood pressure, lower the heart rate and thin the blood. It also has an analgesic effect. The body heat increases too. These combined functions make it less likely the padmasana will become uncomfortable or that one's legs will go numb.
During practice the slight analgesic effect numbs the body slightly. This causes lessened sensation of bliss. But there is a greatly magnified sense of clarity which seem to be inherent in the Kava Waka. So that the practices that build up bliss, on Kava Waka, build up clarity instead. This is an interesting way to practice. The practices that work with clarity specifically obviously are impacted favorably by Kava Waka usage.
The effect is long lasting, so one will experience increased clarity during post-meditation practice as well. There are some side-effects I've noticed. There is a slight sense of nausea which is manageable. There is a slight dehydration and headache that can be treated with ingesting fluids. I've experienced some heartburn which is also reduced by drinking water.
Overall, I would give this substance a positive rating for use by a meditator. It would be interesting what other experienced practitioners' experiences might be with this substance.