We have a panasonic and it works fine, but my advice is to steer well clear of a pbx, once you've got it, it is much more difficult to change to something else. We've tried to downgrade & upgrade the lines etc over the years and it is always a problem.
Plus there is usually the cost of an engineer, I'm pretty good with computers/coding, but the panasonic system is a right pain to set up, let alone if you want get a non tech person in the office to do a simple thing like check voice mail - they have to press about a dozen buttons.
If I could get rid of the existing system (without all the phones being out for days) I would, and I would look into VOIP and virtual/hosted systems, they are much cheaper and offer a lot more flexibility and functions than pbx.