We recently completed a migration from an analog PBX to Jive's hosted VoIP PBX, and are very happy with Jive's service. Jive's system replaces most functionality from our old PBX, while adding some useful new features (better ability to filter/block phone calls, for instance) and an easy-to-use, visual dial plan builder, which I've found to be intuitive regardless of who's using it -- it makes equal amounts of sense to me, the system administrator, and to our sales manager.
Jive's customer service is also top notch. Jive offers virtual fax machines which can distribute incoming faxes to a specified e-mail address. Unfortunately, I made a mistake in configuring the e-mail address I intended to use for faxes, and managed to block all of our incoming faxes on the first day of use. Jive's technical support department combed their logs and send me information on all faxes sent to our numbers on those days, including caller ID data, so I was able to match their information to our own and confirm that we had not lost any sales due to missed POs. They really went above and beyond with this -- I made a stupid mistake, and they dug into their logs to save me, despite a complete lack of fault on their part.
Do be aware that you can probably get a more robust product if you're willing to host your own PBX. But when you host your own PBX, you need someone who can configure and support it, and if it's a VoIP PBX, well... most small businesses simply don't have anyone on-staff who's well-versed enough in VoIP configuration to do so. The support cost of a self-hosted VoIP PBX is worlds higher than Jive's all-inclusive service.