I'm using my 2nd Gen Nexus 7 4G as my daily driver until the Nexus 5 comes out.
I use the GMail app for mail, but then I use Google Apps as my provider, so that fits pretty much hand in glove.
CSipSimple + voip.ms for calls, Viber for IM, Google Voice for SMS, QuickOffice for docs. Works really well. Great for movies on the plane, Kindle App works great, Zinio looks great.
Only gripe is that I need slightly larger pants pockets.