I live in Boston and DS was born in early November. I thought I would need the JJ Cole car seat cover , but honestly, I haven't had the need for it (well, no one bought it for us and I didn't buy it myself). When we go out, I put DS in long sleeve clothes + socks + hat, then I put 2 blankets over him (a thin A&A blanket + a thicker fleece blanket), and he's warm enough even for outdoor walks at 30F. If it's a windy day, I place a carseat cover (those "free" ones I got) to block out the wind. I'd say save your money for something else.
P.S. if you find yourself really needing one after your LO is born, you can always get one then (or buy one cheaply off craigslist.. there are always TONS of them for sale on CL).