I didn't look into this service when we went VoIP as it is new to me. the advantages I see is that it is hosted and ran by the service provider so all upgrades, patches OS issues and such are handled by them. thus freeing up your time to do other items. the disadvantage I would see is the extra cost and availability. what happens when your internet goes down? all your phones would go down too since they have no VoIP switch to connect to. both external and internal calling would cease to exist. also, you would need to make sure that you have good bandwidth to the provider to make sure that the voice calls do not sound bad. depending on how many phones you have this could be an issue. especially since you will be going across the internet and neither you or the provider can guarantee bandwidth when it is across the internet.
we are currently using a Shoretel system with a 16 channel PRI. it is pure VoIP out of the box. simple to setup and easy to administer. if, like us, you have persons that move around to different sites all they have to do is log in to the phone at the site they are at and their extension follows them. also, with a 1U switch at each location if the links between sites go down you will lose voicemail, (if you only use 1 voicemail server), but internal and external calling will still be up and running. our main site has two 1U switches, one for the phone switch itself and one for the T1 interface. then we have a 2U server running the voicemail. the system runs great. also, if like us, your sites are far enough apart that calling either incurs long distance charges the system gives you 3 or 4 digit dialing internally to each office. then if you are calling another business that is in the other city the switch will notice that, forward the call to the other switch and dial locally from there. works great so far!