Cloud telephony — has anyone switched from traditional PABX? Was it worth it?

Our phone system is 12 years old and maintenance costs are spiralling. I've had demos from X-on (Surgery Connect), Natterbox, and one other. Costs are all in the same ballpark. Interested in real-world experience on: call quality when running over broadband, whether the call demand analytics actually help you manage capacity, patient experience of the new system, and implementation pain. Our receptionists are understandably nervous.