I have a fair few Aastra 55i and 57i phones wired up to our corporate Asterisk.
I can't say they lack for features given that they are extensible via a web server and a well documented API. They even come with a set of demo scripts which do 95% of what most people would ever want out of a handset packaged up for several major trix based distros: PIAF, Elastix, Trixbox etc.
As for the built in functionality - they are each virtually a PBX on their own, before you add a "real one".
I'm starting to sound like an advert here ...
No IPv6 on them but then it is near impossible to get an ADSL router with it in the UK without custom firmware.