This feature, better hardware (GPUs in particular for gaming), and NFC payments are all I need for my Windows Phone experience to be "perfect". I have a lot riding on this feature, and if it fails to deliver then I will seriously consider a Nexus because Microsoft has had PLENTY of time to get all this stuff right THIS TIME. No more can they use these "didn't have time" and "need more of this..." type of excuses. They seriously oversold Windows Phone 8 when it first released with a lot of promises, and 2013 has gone and nothing has really happened (minus new apps finally coming, which is a separate issue). The way Microsoft drags its feet with things like this has really annoyed me, so when they do announce stuff like this and it's a bust, I just get fed up with them.
I really hope this virtual assistant isn't lipstick on a pig, with simply more features added to TellMe (though, even THAT would be nice). We need natural language support and to be able to just USE IT without having to remember commands or specific things. When Windows Phone 8 first released, they made the new TellMe API sound nice, where developers could extend commands to their app. However, it's so useless when I have to REMEMBER what the command is for the app in the first place; it'd be quicker and more natural for me to just open the app instead of having to remember the EXACT command. It gets even worse when you have a BUNCH of apps with voice commands, and each app may have a TON of voice commands. Who is going to sit around and memorize all those voice commands.
Microsoft needs to put in the work with urgency that they simply can't keep "waiting" for the next big thing they create and then release it 3 years after everyone has moved on from their platform. They need to release stuff to at least keep up with competition, then worry about blowing them away with new features to add later. Windows Phone itself is already unique, so there is no need to delay things like a personal assistant on the basis of "wanting to wait until we can deliver something so different that it's worth the wait".