We've been using IP phones since the start of 2016 and at first like most people are saying, we had some issues. But after they fixed those issues up, the system has been pretty decent.
We have a couple offices, and now with VOIP we've got all offices on the same system, I can send a call to my office in FL with their 4 digit extension and thats it. Say you have a problem or disagreement with a customer/carrier, calls can be recorded at the press of a button and have proof of what was discussed. (make sure you know the laws associated with recording calls though!)
The system comes with a computer addon that allows us to pick up calls that are on any phone system with a double click on that persons extension.
Seems their support is really good too, and if we have any problems a tech is on site, or someone is calling our IT to resolve the issue.
If you have the bandwidth, I highly recommend the change! We kept I think 1 or 2 bell lines just in case we have a complete internet shortage, and the fax is relayed through a bell line too, but other than that everyone in the office is on VOIP!