I agree with @Matt R.. I live in the valley myself and it is a HUGE area. It would be impossible to discuss the area as one homogenous zone and say a blanket statement about price appreciation. The hills in Sherman Oaks are full of multi-million dollar homes and have seen large increases in value. A lot of celebrities and wealthy people prefer this side of the hill to avoid crowds. Then you have places to the north like Sylmar, where you run into gangs and crime. Even the difference between Sherman Oaks to Van Nuys which share borders is very pronounced. AS far as appreciation, I live in a condo and we're already 10% above the pre-crash highs. My brother lives in North Hills and his home is also well above the pre-crash value. It's true that you can have large changes in value street to street, so having first hand knowledge in a specific area is your best bet to find areas which are desirable and therefore have grown steadily in value.
@Kevin Lefeuvre However, in general it is not as desirable an area for people in LA, I personally have friends that don't ever want to drive "over the hill". However, the area, and more specifically some of the areas, are greatly improving. Along Ventura Blvd we have the same restaurants they have in BH, West LA, and West Hollywood setting up shop here, and we have our own awesome restaurants only found here. We also have a large population of younger people, who can't afford the type of home want in LA, so they buy in the valley. Admittedly, there are some neighborhoods that are bad and will likely continue to less desirable, but we have our own up and coming neighborhoods were people love living and are seeing good growth.