What you'll learn? Hard to say, but you can search "PASS Summit" here on the site and we've usually gotten 2-3 people to write about it.
The conference is Wed-Fri, Fri is not a bad day. It's full of similar sessions, sometimes with a couple repeats of popular sessions. I've blown off days before for various reasons, so it's really up to you.
The thing about Seattle is that there are lots of MS guys and gals there. Most of them can get a day off and bus over, unlike getting approval for travel to Denver/Dallas/Orlando as in years past. Getting a chance to talk to them is really valuable, even if it's 10 minutes. You can't find that expertise elsewhere.
You also get to talk to lots of other DBAs. As nice as the forums are, they're not a substitute for some face time with people.