On May 27, 2001, at 1430 hours Pacific daylight time, a Cessna 177RG, N8287G, was substantially damaged when it impacted mountainous terrain during approach to the Pauma Valley, California, airport. The private certificated pilot and one passenger sustained minor injuries. The owner was operating the airplane under the provisions of 14 CFR Part 91. The personal cross-country flight departed Borrego Valley, California, at 1345, and was destined for Pauma Valley. Visual meteorological conditions prevailed, and no flight plan was filed. The instrument-rated pilot stated that he was descending in visual flight rules conditions, and was on the extended downwind leg for landing on runway 29 at the Pauma Valley airport. He intentionally flew into the top of what appeared to be an isolated, "puffy" cloud, expecting to be in the cloud only briefly and to see the runway again on the other side. The cloud was more expansive than he anticipated and the airplane continued to descend in the cloud for a period of time until the pilot saw vegetation on a mountainside a short distance in front of them. The pilot took evasive action and the airplane impacted upsloping mountainous terrain and slid to a stop through heavy sagebrush.