::sigh:: A thread like this should be stickied somwhere to avoid further repeats.
Different phones for different people... all manufacturers are different, and it really depends on what you're looking for.
Aastra
Best XML, good quality sound, not best GUI
Cisco
Best GUI, better quality sound (g.722 available), learning curve for first-timers
Grandstream
New models are wanna-be Cisco knock'offs, fair quality sound, easy to setup. Special mention for the GXV-3000 which truly is unique, especially in multi-site applications.
Polycom
Best quality sound, ugly phones (opinion), learning curve for first-timers
Snom
Good sound quality (better with g.722), 12 Lines with no side cars, 370 has OpenVPN, most programmable
This is all without expounding into any great detail on any in particular. While I agree Aastra makes excellent phones, I would urge you to look at what you want out of a phone and pick one based on your needs and expectations. Contrary to popular belief, Aastra does not make the best end-all of telephones... the 57i/CT (while great) is ONE OF the best phones, and NOT 'the best'. Who knows, maybe it'll be the best choice for you... but don't limit yourself if another truly is better.
::updated::
I felt the need to point out, that 'this particular phone' may best for your operator... 'this phone' for the boss... 'this phone' for shipping/etc... 'this phone' for the grunts/workers. Don't be afraid to mix & match manufacturers either.
Even within Aastra's own product line, the 57i would be a complete waste for grunts/workers... they could easily use the 9xxx series or 53i if no XML other than Forwarding & DND is required. If they want a directory, then bump them up to a 480i or 55i. This is by no means an endorsement for you to pick Aaastra, but IF it's what you choose... don't get an office of 57i's (unless you've got money to throw).
For the record, I've used them all and today I have a Cisco 7940G on my desk
(we change them out regularly so our sales guys are familiar w/ the strengths & weaknesses of them all)