I've used GrandCentral pretty much since the beginning and it's come a long way. Reliability has improved considerably, though it's still common for calls to mysteriously drop or go silent after about 10 minutes into a call. The call recording, seamless in-call transfer, and unlimited storage space for voicemail and call recordings is very nice.
A recent enhancement to the service allows you to forward incoming calls to a Gizmo Project/SIPphone SIP number in addition to PSTN numbers. This is quite valuable for someone who would otherwise be paying $12 or $36 every 3 months for a PSTN number from Gizmo. Hopefully this capability will expand to other SIP providers in the near future.
If there's one drawback, it's that product support is very hit-or-miss right now. Of course, it's in beta, so some problems are to be expected. However, it seems to be a matter of flipping a coin as to whether or not you'll get a reply to trouble tickets and support requests. I've got several in that have gone ignored for weeks, while a previous trouble ticket about persistent downtime in Richmond, VA got a personal phone call from Craig Walker, CEO of GrandCentral. Seems it's either all or nothing. I'm sure that will improve with time and the official release of the product, which appears to be right around the corner.
Even with its occasional glitches, it's been reliable enough lately to be my primary phone number. I've gone strictly VoIP (more incredibly, almost entirely Wi-Fi VoIP). The Gizmo integration is fantastic, since that's what I use for my SIP service (you can now customize your Gizmo Caller ID, so I have mine set to transmit my GrandCentral phone number). I think GrandCentral's feature set-- particularly call recording, blocking, custom call handling, telemarketing filters, etc-- are particularly valuable paired with something like Gizmo.
All that said, I'm certa> Has anyone tried the service offered by Grand Central?