Running with a weight vest, particularly if the weight balances around your center of gravity, can be a help. I've known some people who felt they got a lot out of it. You might find it particularly useful uphill (though you would have to take any downhills especially easy, to avoid impact injuries).
Each running step has both a vertical and a horizontal component. To keep from falling on your nose, you would adjust (probably without even thinking about) to a heavier bodyweight by slightly increasing each step's vertical component. As someone pointed out above, this would risk altering your mechanics.
However, I still think the added weight might be a help overall--beyond just the uphills--if you built it up gradually, and if you followed each "weighted" run with some unweighted acceleration runs (pickups, strides, shakeups, whatever you call them). These would reinforce your normal mechanics at something roughly approximating race pace. Similarly, before a repetition workout you might want to run your warmup with the vest.
I second the emotion of whoever said to avoid ankle weights. I tried them and got severe calf strain, strangely enough.