I have a considerable amount of hours flying sailplanes in the Sierra mountain wave out of California City and Minden. You almost always have to tow through the rotor to engage the wave and much time is spent with the controls at the stops, keeping the tow plane in sight, and flying away from tow line loops. But, once you're actually in the wave it's all worth it, smooth, serene, and a view that seems to go forever.
It's not the wave that typically bites GA planes, it's the rotor; and it's usually visible when the wave is strongest because a really strong wave system usually has a moisture component to it (but not always). Watch out for a cloud that appears to be tumbling on the lee side of of a mountain range when the wind is perpendicular to the ridge and a stable front is blowing the wind across the ridge tops at anything over twenty-five knots. That cloud is showing you the upside of the rotor and it will start to disappear as the rotor starts to turn to the downside. Always try to be somewhere near maneuvering speed if you think you may be anywhere near the rotor, as it can be extremely violent there. The wave is usually just above the rotor zone and it's the best place to be if you're trying to transition the wave across the ridge either upwind or downwind, because it's almost always smoother by an order of magnitude - this is where the Lennies live when moisture is present. Best course of action is to engage the wave and cross the ridge at an angle to the wind and the ridge to maintain a forward ground track and use the smooth lift to your benefit by pushing into it. Be mindful of your true airspeed and don't exceed your true airspeed redline.
I've crossed the Sierras twice in GA planes during pretty strong wave. Once in my Maule and once in my RV. My encounter in the Maule was exciting even though I was prepared for it and and I thought I was above the rotor...I wasn't. Fortunately I was already flying maneuvering speed because I knew that I was in the rotor zone, but it was still a wild ride. My wife and I flew the RV-6 through the Sierra wave in March in the same area that Gary described his encounter. The ridge was capped and the rotor just barely visible. We climbed above the rotor and flew diagonally through the smooth lift and sink across the Tehachapi Valley and descended down the upwind side of the Sierras to our destination in the San Joaquin Valley. The transition was very smooth, but the wave lift and sink was quite strong - the primary was cooking at 1500fpm. We also encountered wave conditions near Alamosa, CO on our way to LOE this year. As we came southward down the valley we got a couple of sharp jolts in the seat. I looked ahead and could see the rotor clouds forming ahead of us so I moved over to the east side of the valley out of the rotor zone and the ride smoothed out considerably. Flying through that rotor at RV cruise speeds could have been very unpleasant to say the least...
Wave is generally characterized by a unstable layer of air on the bottom where the rotor resides, a stable layer with steadily increasing wind speed with altitude where you'll find the wave, and another less stable layer above. The wave bounces off of the unstable layers above and below like a spring, but the lift and sink are very smooth and usually not very violent in the transition zone between lift and sink. The lift and sink in the wave can, however, sometimes exceed 3000fpm.
Something to keep in mind when crossing a ridge under known wave conditions. These sorts of weather conditions are most prevalent during the Spring and the Fall, but they can occur at any time of the year.
Bottom line, pay attention to the weather forecast signs and the immediate environmental sign posts for mountain wave and you'll have a much more comfortable and safer flight. I think most bad encounters with mountain wave occur because a pilot is trying to stay under the wind and ends up flying smack dab into the rotor at high cruise speeds.
