This attraction has quite a bit of history behind it and all the commercial and historical significance aside, its just a great place to walk around and enjoy with family & friends.
I am inclined to say Summer would be a better time to go there for obvious reasons, but again, i am not from Chicago and i have never been here during winter so i cannot vouch for your winter experience.
My take on this attraction:
Allow about 4 to 5 hours to leisurely enjoy the place.. its huge.
Try not to drive if you can.. if you do, be prepared to pay a few bucks for parking
[i think its around $ 30 for the day during summer time]
Not too far from most of the other attractions in the City
Take the Ferris wheel to get great views of the City