THE PLT WAS OPERATING AN ON-DEMAND AIR TAXI PASSENGER FLT TO SANTA ANA, CA. THE ACDT OCCURRED DRG DESCENT, WHEN THE ACFT COLLIDED WITH A MTN AT 2060 FT MSL. THE PLT HAD RCVD A PREFLT WX BRIEFING IN WHICH HE WAS ADVISED OF LOW CEILINGS AND REDUCED VIS IN THE LOS ANGELES BASIN, SURROUNDING MTNS OBSCURED BY CLOUDS, AND THAT VFR FLT TO SANTA ANA WAS NOT RECOMMENDED. HE DEPARTED VFR. WHILE EN ROUTE, THE PLT WAS ADVISED THAT SANTA ANA WAS REPORTING 1400 FT OVCST WITH 5 MILES VIS. A VIDEOTAPE RECORDED BY A PASSENGER SHOWED MTN PEAKS PROTRUDING THROUGH A SOLID CLOUD LAYER AND SHOWED THE ACFT DESCENDING INTO THE CLOUDS. WITNESSES DESCRIBED A LOW CLOUD CEILING NR THE CRASH SITE AND CLOUD TOPS AT 5000 FT. EXAMINATION OF THE WRECKAGE REVEALED EVIDENCE OF POWERED FLT AND NO EVIDENCE OF PREIMPACT CONTROL OR ENGINE MALFUNCTION.RECORDS INDICATED THAT THE PLT HAD ENCOUNTERED IMC ON ONLY 1 FLT IN THE PREVIOUS 9 MOS. HE WAS DIR OF OPNS FOR THE OPERATOR.