I have a friend whom runs a Toshiba phone system dealership, and he also sells ESI. I used to work for him on both types a few years ago. ESI is easy to use, easy to set up, but not very sophisticated in it's feature sets. I would look at it against other systems, and know, they are fairly new at supporting anything above a small number of extentions. They can support recently many more exts.. but I do not know their track record in that.
Get some references from your ESI vendor for your size application, 3 years after implementation is adequate to give a reference, 2 years minimal.
You do not always get what you pay for, but you never get what you do not pay for.