Well .... my understanding was it is windproof (all but the heaviest winds) and could keep you warm. It doesn't seem to do either just around the house. I can't test it outside as it is about 85F with no breeze. I will need to wait to test when we go to the mountains in about a month.
I have a rain poncho to go over myself and my pack.
The jacket is meant to be used as a windshell. It is not meant to be an insulating layer, but to extend the effectiveness of your insulating layer. It will allow some airflow in order to increase breathability so that perspiration doesn't soak your insulating layer.
For effective layering when the weather turns cooler, using a three layer system allows extreme flexibility to regulate how warm or cool you become while you are active or resting. The idea is to add or shed layers as you heat up and cool down during activity and rest. One wants to control soaking their insulating layer so that you don't get chilled while resting or ceasing activity.
1. Skin layer. Usually a medium or lighter weight shirt or thermal top that is made of either a synthetic or natural fiber wicking material. I prefer merino wool, others prefer synthetics. Both do a good job, but merino wool doesn't develop the stink that synthetics can and thus can afford skipping a daily washing routine.
2. Insulating layer. Depending on the expected weather conditions, this can be a lighter or a heavier fleece, a puff jacket, or a synthetic jacket rated for the expected conditions. This insulating layer should have the ability to "breath" well.
3. Windshell. That's what the Montbell jacket is. I use a Patagonia Houdini, which is the same basic principal. It is a seemingly contradictory thing that this layer would be somewhat air permeable and yet effective at being a wind blocker. However, it will effectively increase the temperature range and comfort of your insulating layer.
When you walk, you can combine any of the layers to keep warmer or cooler. The skin layer can be used by itself as a shirt. If things get a bit chillier, then you can add just the Montbell. If it is really chilly, use the skin layer with just the insulating layer. And if it is really chilly, use all three layers. As you walk and are exerting yourself and are getting warmer, remove layers to reduce the amount of sweating that can soak your insulation.