Are convertible tops (not the hard top) able to keep driver warm in winter months (this is with the top up, of course!)?

Yes, the tops are designed to seal and hold heat/air. Unless you have a problem with how it seals, you'll be fine.

oh yes, infact if its on recirculate i find myself opening the window with it being close to 0 degrees F that heater really kicks in the small cabin!

Yes, the Miata heater does a great job, even in very cold temps.

yeah they're great, though i normally drive mine with the top down, as long as its not raining or snowing, lol

Absolutely the soft tops work fine! I live in Northern Alberta and the winter temperature here has been minus 20 to 30 C, and I've been turning the heater down because it's cooking me!

because of the small interior, the heater quickly warms up the car and does a great job. I don't have a hard top and I was perfectly fine in sub zero temps. Only problem I had in cold temperatures was that the car would buck a little when engaging the clutch for the first 3 minutes of driving or so... this isnt a major problem and many miatas do it esp. in cold temperatures

i have both hard and soft top for my miata and the soft top definatly keeps me warm

My Mazda MX-5 2.0 2008 have heated seats that almost makes it unnecessary to turn on the aircon...