In my opinion, for the people that have had little-to-no success with Kava-Kava, you're either using the crushed plant material (that usually doesn't contain as much of the "roots and bark" material as it should), or you're simply not using enough.
It's a damn cheap plant and pretty easily found in most herbalist stores, but these places usually sell the poor quality commercial crap that doesn't contain a high amount of Kavalactones (ie: the psychoactive that induces the sought out intoxication).
Ohh and for the record, Kavalactones are thought to act on the GABA neurotransmitter in the brain, which is responsible for the sedating and calming qualities of the high. I also read somewhere that there's thought to be Dopamine ataganonistic effects in the brain.
If you do make the tea successfully, I highly recommend not skulling it like the Fijiian natives try to encourage, as this is probably why you're feeling sick and nauseus which is possibly over powering the enjoyable buzz produced by the tea. Drink slowly. It's an acquired taste, it really is.
In my experience, mixing it with alcohol will tend to make you feel more rotten in the morning, much more so than if you were to just drink alcohol alone and there really shouldn't be any lasting effects into the next day. What I experience is a mild (similar to an Opiate afterglow) refreshing awakening, that leaves you feeling peaceful and chilled for the rest of the day. Nice feeling.
If you feel the need to mix anything with Kava-Kava, I highly recommend combining Cannabis, we all know that weed is pretty well reknown for having great synergistic qualities and can really bring out the intoxication of a mild psychoactive.
I think the main thing I enjoy about Kava-Kava, is it's muscle relaxant properties and the clarity that ensues, not to mention the mild visual "glowing" effect, not dissimilar to the one produced by a low dose of alcohol.