As someone who builds and installs large phone systems for a living, I cringe whenever a customer tells me "Yeah we've got a T1, coming in over Time Warner."
A traditional copper PRI from Verizon is the ideal service I like most of my customers to have, I never get anywhere near the same level complaints of call quality issues or service outages for a traditional PRI that I get for any PRI coming in over the internet. Well, except after hurricane Sandy, after that storm we had a number of customers switch over to an IP based PRI or a pure SIP solution. It made sense since it took Verizon months to fix their wiring, but a lot of these customers that switched wanted to immediately switch back as soon as Verizon was available again since the quality was so god awful.
I have no problem with Verizon using fiber and IP based telephony in the back end since I they're not going to be able to maintain their legacy equipment forever. But, don't send everything down the same pipe and just install a $200 Adtran on-site and expect it to be anywhere near as reliable. Especially since a lot of the support engineers for these carriers have no idea how to do anything with an IAD. I've had support engineers tell me I need to send a SIP redirect to forward calls out with the proper caller ID, well sure I'd love to except I'm being handed a PRI and the SIP side of things is all them.
Anyway, for customers that have rock solid internet and a separate dedicated pipe for a SIP trunk, I have no problem going native SIP all the way to our equipment. My problem is when someone out in the boonies thinks they'll save a ton of money switching to VoIP service from their cable provider. Instead it just means dozens of billable hours trying to explain to this customer that while their internet service is excellent for checking Facebook, good voice quality requires a solid internet connection with little to no packet loss and very low latency and nothing we can do to their PBX will change that. Although as one coworker pointed out, as the number of people who grew up using cell phones all their life increases, the less complaints we will receive. People who are used to POTS lines are going to be used to picking up a phone and having excellent call quality, people who grew up with cell phones are much more accustomed to jitter, echo, and poor call quality so I'm sure they'll be fine in a pure IP telephony world.