Dewa Sanzan in winter

I was thinking about visiting Dewa Sanzan in mid February. Is it worth going to then?
and also, is it easy to get to from sendai?

I don't recommend it as two of the three shrines are inaccessible. But I imagine that it could still make for a nice experience.

If you actually go, I recommend to stay overnight at the "temple lodging" just next to Haguro shrine and participate in the shrine's morning ceremony:
http://templelodging.com/spot/01tohoku/dewa005.html

is it easy to get to from sendai?

I would not call it easy. But it is not that difficult, either. You should take a bus or train from Sendai to Yamagata and then a bus from Yamagata to Tsuruoka. From there, it is another bus ride to Haguro-san, the only shrine that is open in winter.