I've been using GrandCentral and Gizmo for about a month now, and I recommend everyone check them out. Why Gizmo and not Skype? mainly, because GrandCentral integrates directly to Gizmo without going out to a "PSTN call in number" and back.
GrandCentral is rockin'! OK, their "Beta" is a genuine beta -- I've had some calls drop, calls not go through, voicemail never pick up... they have some bugs in the software and some issues in ops. But it's still a fabulous service. And when something does go awry there's a real live human on the other end of a chat widget. She's helped me a couple of times and is a real pro. Not to mention the longer the beta goes on, the longer you can enjoy the full power of GrandCentral, including unlimited calling into the PSTN, for free!
The "one phone number to rule them all" concept has been around a long time. The question is when to stop being a skeptic and when to start believing. Photorealistic graphics, good color printing, network-based apps with rich interactivity... all were promised several times and several years before becoming everyday reality. I'm not perfect at figuring out just when "this time around" becomes the time it really works. If I were, I'd be a VC instead of a developer. But I believe the time is now for VoIP and advanced apps for VoIP. Why? Besides Skype, and corporate adoption of VoIP, I think when Verizon sues Vonage over questionable patents in order to put the brakes on, and people actually care, then the time has come. Just like mobile apps had clearly crossed the chasm when everyone worried about NTP shutting off their Blackberries.
I tried to go "all in" by making the GrandCentral number the only one on my business card (due to a clerical error it didn't completely work). With my desk and my cell phones both ringing for business calls a client or coworker would be guaranteed to find me whether I'm in a conference room, at my desk, or out for a coffee -- plus I wouldn't need to check my office voicemail remotely, or force a client to deal with "I got voicemail... I'd really love to get feedback on this issue now, but is this really important enough to call his cell?"
For me, the remaining question isn't whether these services can be hits. They can. The question is how to manage an enormous number of communications accounts each with its own fees. I have a landline (originally for DSL, now because it's still cheaper for local by nearly an order of magnitude than cell or VoIP calls); a cell phone with minutes and a seperate data high-end data plan; a Gizmo phone that offers some free calls, but mostly charges 1.9 cents per minute (if you're keeping score, that's lower than a cell phone but higher than a landline); and GrandCentral, which is free in beta but plans to charge for connections into the PSTN. And this doesn't even include the cable modem account that gets IP to my house.
Each of these services plays a specific role and they all dovetail together to keep me connected to pretty much whatever I want. But there has to be some way to simplify this picture for mass adoption. Unfortunately, the only players in a position to do this are large telcos or cable companies, and they aren't known for innovation or customer service. In the Bay Area, for example, Comcast -- which offers "bundles" of cable Internet, TV, and VoIP -- bought radio time to hawk photo sharing as their great new thing. This is San Francisco in 2007, for godsakes, gimme a break. I'm not taking sides with AT&T either, who would also probably be happy to take $200 a month from me to sell me around half of what I need.
The other question is how number portability applies to the new services, in case this time around turns out to be more like the last time around.