OK, so I have one of these.
Why did I want one? The company I work for has all its phones with VF and there is no signal at my house (1 bar max which tends to go to zero as soon as you try to use phone). Forwarding my mobile number to my landline or using Skype etc is not practical.
How do you set it up? Plug it into the mains and into a spare port or your ADSL/Cable router. Set up account on VF website and register the device (serial number, postcode, what floor it is on (!) and a list of the VF numbers to enable). You then wait for a number of hours for it to connect back into the VF network. There is zero indication as to how this is progressing. In my case after 12 hours it had still not connected, a call to VF support resulted in a set of (generic) additional instructions for punching ports through my router. Three hours after doing this it connected (but I have no idea if it was really required but suspect not) and I got a text message to to alert me of this.
Does it work? Yes, I have 5 bars in most of my house. It seems to be binary though, it's 5 bars or nothing and if you are on the borderline you risk the phone handing off to the main network and the call dropping. I need to experiment with this. Data services work fine.
The main issue I have is the length of time it it takes to connect the first time, I think the VF support line are going to get very busy if this thing takes off. The least they need to do is have a website with configuration settings for common devices (such as mine which is a bog standard BT supplied unit). Quite why the box doesn't publish more details status information via a simple web server is a mystery to me.
Would I recommend it?
If you are tied to VF and have poor/no coverage then £50 is not a huge amount to pay. Yes, perhaps it should be free but I'm sure that would come with conditions and I certainly don't want anyone elses voice traffic going down my broadband line....