I ran the I Zens last winter for the first time. Definately a good bang for the buck. Got some mounted on 15" wheels. They were great in un-plowed snow (6-8") and packed snow. Didn't really get much of a chance to try them out on ice though. But, they do have sipes on them so they should make a decent ice tire. The nice thing was the fact that I put around 5k on them and they showed almost no wear. And that's with 1.75 front camber, .75 rear camber and 1/8" toe out up front.
Auto-X-ZTS is right. You definately don't want to put just two snow tires on the front of a FWD car. What you need to think about, is that although forward motion traction is important, it is more important to have overall stability for braking and turning in order to keep you pointing in the right direction. If you have to get on the brakes hard, the fronts will bite and the rears will not. If you're not going absolutley straight (and maybe even if you are), you'll probably end up pointing the wrong way. Also, if you're turning and the fronts bite and the rears don't, the same thing is going to happen. Perhaps even quicker.
If you can't afford four snow tires this year, I suggest going to the local junk/salvage yards and try to find four stock 14" or 15" wheels with some decent tread left on the tires. A skinny all season should be much better in the snow than the 712's in a 205/50-16 (I'm just assuming that's what you have). The skinnier the better.
On the other hand, I've had pretty good luck with running just a pair of snow tires on the back of RWD cars. It still doesn't help at all with braking, but it does give you the ability go get moving when you normally wouldn't be able to. And its relatively safe with the snows on the back since it actually reduces oversteer. But you still have to keep in mind that although you were able to get moving, the chances are that you are going to have a tough time coming to a stop.
I've put 7500 miles on these tires and they look new (other than the road rash one suffered being forced into a curb at 30).
They drive decently on dry roads, but you'll notice the difference between that and your stockers. They work extremely well in snow and ice; I can't outdrag the 4-wheel drives in the snow, but I can outhandle and outbrake them without a sweat (especially since they rarely run snow tires). I have no problem going through foot-deep snow on these, though they're in their element on hard-packed snow and slush.
Whatever you get, make them as skinny and tall as possible; the Focus likes to hydroplane in the slush on wider treads.