As someone who has been on the traditional PBX system, to VOIP (to another VOIP) and then finally moved back to a traditional PBX.
I was on VOIP for about 4-5 years. When it worked great, it was fantastic. In addition to the other's comments, I liked the flexibility to go in and change greetings remotely very simply. For example, if we were closed because of bad weather...I was able to change the greeting from my house so customers would know why we weren't answering. Our traditional PBX system lets us do it, but it is more of a headache. The dial-plan system was very cool to use. However, I switched back and I'll detail why. If you decide to go with VOIP, here are two big important things to consider:
1) Go with a reputable company. We used Nextiva the first time around, and I think that company is run by a few teenagers out in the Southwest or something. Every tech support guy sounded like he was 18 and the support there was terrible. When we started having call quality issues, they could not get it figured out. We were with Jive the past few years and they were miles better.
www.getjive.com. If I was going back to VOIP, I would go with Jive without question. Great support staff. Very slick software. Only had 3-4 total outages the 4 years we were with them.
2) Make sure your internet is capable and consistent enough to handle it. No internet, no phone. If you are in a large city with upgraded internet...then this probably isn't a worry for you. However, I would make absolutely sure your internet could handle it. We are in a rural area, but still have high speed AT&T DSL (15MB).
3) Make sure your modem/router is compatible with the specific phones and company you will be using.
Sometimes during high traffic times (if a lot of bandwidth was being used at once), our customers might say that we sounded choppy. The tipping point was when AT&T forced us to switch to UVerse. It was still a DSL line, but a different technology. We had to get a new modem/router, etc. It didn't "jive" with our VOIP phones. We went through tech support after tech support and could not get it to work correctly. Customers would constantly say that they couldn't hear us, or that we were very choppy sounding like we were on a cell phone in a bad reception area.
So I finally bit the bullet and went back to a traditional PBX system and I don't regret it.