I have an iPhone 4which I use for Twitter and to some extent Facebook. I also use Google calendar which is synced with it. As for apps, I use Trip adviser, a QR reader, Recorder pro (for podcasts etc), Area finder(uses phone GPS to measure areas by clicking at perimeter points), eBay, Camcard(imports bus card details from photo), Mycarcheck(VRN checker for chasing poachers etc) and BBC news. Weather via Met office.
Battery life is OK but will not make two days.
Synced calendar is great and others are handy and easy to use. All free except Area finder which was about a fiver.
cheers
FarmrPhil