I grew up on the East Coast, but I love Silicon Valley. Not necessarily as much as I loved NYC, but they're very different.
Silicon Valley has really, really nice weather, almost to the point of monotony. It rained something like 3 times last summer, and the rest of the time it was 75-85 degrees without being too humid. There's a lot of asian cuisine available, and some of the best restaurants (of many varieties) in the world to the North in Napa. Nature is extremely accessible, and some of the best skiing in the US is a relatively short drive (the Rockies are much, much better than the Appalachians). It's a day-tripper's heaven.
As for downsides, Houses tend to be smaller, and even those are absurdly expensive. Rents in the better parts are just a cut below low-end NYC rents. The atmosphere could be described as "Irrational Exuberance" or more commonly as an echo chamber, which can be great encouragement if you're trying to do something crazy, but it can also severely weaken your BS filter if you're not careful. Silicon Valley itself is better-than-average suburban sprawl, but it's still suburban sprawl. The public transit system is lackluster compared to NYC, but it is an option, at least. Access to a car is generally necessary (ZipCar might work, though).
There are a bunch of other upsides and downsides, but those are the impressions off the top of my head from having lived there a bit less than a year. Overall, I really like it.