I've worked with 3-4 of these firms in the past. My general impression was that they didn't provide significant help in the early stages and in making connections to investors, etc. Also, how much networking support you get out of them varies greatly from partner to partner.
Just so you know, it's really not that hard to make connections if you live and work in the Valley and attend any number of social events where people interested in the startup ecosystem congregate.
As far as the legal work -- in the early stages most of it is boilerplate. And the senior partner you meet with will, understandably, let a junior associate do most of the heavy lifting. If you have the know-how and motivation you can save yourself a lot of money by starting with something like Orrick's Startup Forms Library or let a small or solo firm set up your business structure.
The big-name law firms are much more useful in the later stages when negotiating complex deals or when you are about to sign a term-sheet with a VC. When it comes to intellectual property they will also refer the business to boutique IP law firms -- something you can do yourself.
Most of them, incidentally, consider giving a starving startup $10-$15K worth of legal fees -- most of which will get eaten up in setting up the corporation/partnership -- in return for a right to invest, if they like your idea and think it's worth the risk.
My suggestion is to do the basic boilerplate stuff yourself or with a small firm. You can always approach the big name ones when you have a specific need and can afford them.