Bought a Castelli Gabba II last year and loved it - amazing fit and I used it for everything from 25-50 degrees weather. (I ride hard all the time) At some point I thought I'd like a warmer jacket for colder weather and days I go easy. Here are the pros and cons of Alpha jacket: overall, it's not as versatile and as great as the Gabba II.
-extremely warm, on days at 25 degrees (15 with wind chill), I still felt almost too warm with a Patagonia long base layer. For anything above 32 degrees, all you needed is a thin short base layer.
- fit not as good as Gabba II, it's a little baggy. I wear XL in Gabba II and bought both L and XL of Alpha, kept XL because L is too short (although fit was better) at stomach.
- not ventilate as well as Gabba, so I tend to overheat, even after unzipping the outer layer.
- very heavy, you can feel it.
Overall I think it's probably the warmest winter jacket, but if you are like me like to ride hard, this is almost too warm for anything over 32 degrees, which you can layer with the super awesome Gabba II jacket, which I gave a 5-star reivew.
I am keeping my Alpha jacket, despite it didn't reach the same level of versatility and awesomeness as the Gabba II jacket.