i have not installed shoretel yet, but we do sell it. the guys that have installed shoretel tell me featurewise it is just a little leaner than the ip office which i do install, and service. it is however an install, and walk away system with little to no issues with it, leaning toward the no issues heavily. we have numerous customer endorsements of the shoretel, single, and multiple site systems. these are guys that are experienced with multiple manufacturers systems, and multiple systems from each manufacturers.
we are in the largest 1 percent of avaya houses, and probably the same with shoretel. i can not recomend it from experience, but the company mind trust is impressed to the point that we are presenting the shoretel to all customers as a highly trusted platform.
also, forums are the place to hear primarily the headeaches, and shortcomings of systems. the lack here, does tell you something, if you listen.