Figured I'd give my 2 cents from the customer point of view. :-) I am an end user (evil IT guy) who recently ended up purchasing a Mitel 3300 (all IP) over an ESI. The ESI was priced great, had some neat features, layout was nice, etc etc., however it seemed to be somewhat limited, ie-you have your choice of...one phone model, one software package, etc. The Mitel seems to be more of a platform that can be integrated in more ways than one, offers a plethora of phones and accessories to pick from, and it seems like they're more with the times when it comes to integration with new things like the Microsoft stuff.
A couple other things that stood out with the Mitel were that their phones use a base for their networking end; you can take your existing phone and put it on a wifi base, or a gigabit ethernet base (to piggyback with a computer with gigabit). They also have some tie-in with the new Microsoft Office Communications and all. To us, it seemed like a more well-rounded system.
I felt bad because our existing vendor (a small shop), I was very comfortable with them and wish we could have gone with them, but honestly I feel we're getting more bang for the buck with the Mitel. It was more the system than it was the vendor in this case. Its one thing to say that this system or that system can offer everything the customer needs (to make a phone call), but for us we looked at the whole picture.
