IMO - if you are a large enterprise or a large SMB then I think you can afford a proprietary IP system from a major player instead of an Asterisk based system which I feel is for small SMB's looking for low cost bells and whistles.
(I am sure that will upset a few members).

We have done some with Cisco, Ericsson and soon Siemens Open Ofice and 4000 - Open office is really nice. I have heard good things about Shoretel - the only issue I have with Shoretel is I believe it is server driven and anything you have to ctrl-alt-delete to keep running is an issue in a large, larger enviroment etc.