I need good Exchange syncronisation for my smart phones. I was never a fan of the IOS email app, but its still better than the Android stock option. While email is kind of reliable I still havent found a working calendar integration. I just want to be able to accept or cancel meeting invites when I am out of the office.

I tried most free apps (including K9) but all other apps cost about $30, I am not keen on wasting money on another not working app. So do you guys know a good email/calendar app?

Needed:
Good Exchange integration
Support for several accounts: Exchange, IMAP, Gmail (unified inbox not needed)
Working calendar integration (ability to accept meetings on the go)
A GUI that doesnt suck
Decent configuration options of sync frequenzy

For these kind of options I would happily spend my money, but I havent found anything yet.