Personally I use my phone quite extensively to search for routes (google maps), to check my mail, to chat, take photos, play a few games, glance at the sync'ed calendar, listen to music, browse HN and... to make phone-calls.
Not a single time have I felt a need to search something from my "social graph", or my dropbox, or my calendar, or my facebook, or anything like that...
I often need to search for a message in my secondary account (I'm logged into my primary acct in Safari on my iPhone). Before this, the options were:
1. Log out of primary account and log into secondary account (laborious, especially if you use 2-step auth) and search in the gmail webapp.
2. Search using the iPhone mail app (awful).
(Makes me miss Android's native gmail app.)
Greplin searches both email accounts and tons of other useful stuff.
I have _many_ different email addresses, but they all (except of the one at work, for obvious reasons) forward the mail to main one, which labels them for me to know their origin.
I discovered this about 4 months ago and it's made my email life so much easier.
In the browser, we've had great luck with Google's multi-accounts feature:
http://www.google.com/support/accounts/bin/answer.py?answer=...
While email search can be slow and cumbersome at times it's what I would consider "good enough".
I haven't used the app though and it might change my mind completely. I'd love to not have to open an application to start doing something. I'd like to speak an address or say a name or start typing a number and the phone be able to guess what my intention is and provide the best options. Sounded like that's in part what it does --- but I'd like that to be a native function of the phone, not provided via an app. Wonder how doable that is on Android. Presume not at all on iOS.