Valve should probably be considered a very serious competitor in VR. I'm quite surprised actually, as they have repeatedly claimed that they aren't working on commercial VR. They did create a very expensive demo unit that has been regularly talked about as the best way to show the potential of VR at the moment. That unit is a bit more advanced than the current Rift, and it's interesting to see that they're taking some of the ideas from it into their commercial unit. Particularly, the ability to move around physically is the most notable feature of their demo unit, and this commercial unit will support that as well.
It does sound like Valve's VR unit will be expensive though, and I suspect Oculus might beat it on price and quite likely also technical specs. We'll just have to wait and see.
didn't the Rift start at Valve and Valve let the developers go after deciding not to pursue it?
No, the Rift prototype was made in Palmer Luckey's garage, and he certainly didn't work for Valve.
I'm thinking 2016 or 2017 for it to truly hit mainstream. It took a year for the XB1 and PS4 to semi-mature and get people on board, and I think this is an even steeper adoption curve. It's all going to be about the software, and it's going to take time to make those "killer apps" that get the general public on board.
You're more optimistic than Oculus, then. They seem to be expecting something closer to 5 years before VR goes mainstream. They're aiming for technology enthusiasts at first, and would be very pleased with a million sales during the first year.
Also, remember that Oculus has acquired quite a bit of software development talent. They're working on things behind the scenes. We should expect some impressive software releases for VR at the same time as the Rift launches, even if we haven't heard of any of it quite yet.