We had started down the road of ESI eSIP and actually have a few installed... They have been more trouble then they are worth in my opinion. ESI's Communication Servers were actually very robust and user friendly, but the eSIP is one annoyance after another. Being an ESI dealer for over 20 years we toughed it out a while before we decided this was not a product we wanted to continue with.
We also carry Avaya IP Office, although it's been a few years since we installed one... their OK, but we believe they just don't compete with modern phone systems.
We also looked at SwitchVox, Grandstream UCX, and Allworx... Had some demos with two of them (not Allworx, I don't know why exactly) and they were OK, neither one seemed too want to do more than give us a 30 minute "light show" and find out our sales numbers... But Zultys took the time to listen to us, and their demo with it's desktop and mobile integration and seamless mutli-site networking, literally blew us away from the beginning, and then they took time to do a technical demo and took an afternoon to go through the system features in depth from not just the user side of things but from the perspective of setting them up and how they work on the backside as well. They have just been great to work with all around. All of our technicians are certified on the product as well, at no charge from Zultys. The programming is pretty straight forward once you get the flow of it, although several things require you to "bounce around" in the programming that could get confusing if you haven't done it before, but the flexibility is really good.
Gonna be honest, we haven't done an install yet... We have a couple proposals out there, one for a car dealer with about 60 phones and another for a small community hospital with 3 satellite clinics (all in areas where lines can't be ported away from the local telcos) totaling around 100 phones... So I might change my mind after a couple installs, but so far we feel pretty confident in their product. We don't do a lot of premise based systems anymore, maybe 4-6 per year, but focus on our own hosted platform which is getting close to 10k devices now.
My previous employer (I left my current employer for 2 years then returned) in the Minneapolis metro areas has been carrying it since shortly after I left and has done several dozen installs... I have spoken to some tech friends I still have there several times and although they had some odd issues early on, it's been pretty flawless for years now.
Curious why you say it "isn't a good system"?