YANBU. We have done this for a few years now.
For me, the main motivation isn't saving money, but avoiding the rush. I have social anxiety and going to the shops when they are crowded with stressed out shoppers is a guaranteed meltdown for me.
I do a lot of it online although I am going to make more effort to support a local independent toy store this year. But for online stuff, I will try and have it done by December 1 so I'm not worrying about snow, postal strikes etc. I keep wishlists on amazon for present ideas.
The main disadvantage of starting early, for me anyway, has been the risk of overbuying because I see more things that they'd like as the months go by. I have had a huge change in mindset this year though and it will effect Xmas - basically for the last few years we have gone overboard due to guilt (DH injured, me becoming disabled, basically a really shit couple of years) and while our DCs do appreciate and play with their toys, our house is just too cluttered and we are making a big effort to simplify our home, and not have so much Stuff coming in, so we will be more selective.
Although actually we have got their main present already as I found out that playmobil won't be making the pyramid anymore, and DD is fascinated by ancient Egypt. I am also putting together a big craft stuff kit so I will keep an eye out for stuff to put in (eg going to the fabric shop for remnants rather than end up paying over the odds for precut foam shapes in ELC, etc.) - I am quite excited about doing this 
So far the DCs don't really ask for anything particular (they don't see adverts on tv as we only use it for DVDs) and we can just choose a few things we know they will love. We don't have a big family so basically they only get gifts from us, DSCs and my parents (but the latter is chosen by us anyway as they don't like choosing).
I don't wrap until December as I found it gets tatty if I do it earlier, and plus I love wrapping on my own with some carols playing etc. But mostly I want December to be free for lazing, cooking, carol services, crafts, walks in the woods collecting pine cones, and our new pre-Xmas tradition of a family trip to the movies and then a pizza lunch (did it with just DSDs and DD last year but now DS is old enough he and DH can come too).
Sorry for waffling 