Tehachapi is not bad coming from east to west. It is a steeper climb coming from Bakersfield. The fact that it only reaches 4000 ft can be a little deceiving. Bakersfield is only about 400 ft elevation so it climbs from there to 4000 ft in about 30 miles. It is also traveled heavily by trucks and is only 2 lanes. Again coming from the east it is less of a problem. Coming from the west at times the trucks can be a nuisance. They use what I call the 3 blink rule on the way up. They turn their blinker on and on the third one they are coming over regardless. The CHP tend to work the downhill run from Tehachapi to Bakersfield pretty heavily. At times they will use a plane and have cars sitting at the bottom pulling people over. There is a bit of a strait stretch there where people tend to let their speed build. All in all though it is not too bad just watch your speed on the downhill.
A couple of folks have suggested 101 as an alternative. You can either take it strait through LA or as suggested take 5 up to 126 and across. Time of day will really come into play. Personally I like 101 as far as Morro Bay. From there to the Bay Area just depends on what you are driving and how comfortable you are. Lots of discussion on this board about that route.