I read quite a few reviews before finally watching this with my almost 3 year old. I admit it was borderline as to whether I was going to watch it or not. In the end I'm glad we did. To some of the points previously made, about it being scary. It is talking about a very valid fear that every child has, that of separation. It introduces the fear from the perspective of the animals, and shows that the way you see the world can change based on whether you are scared or not (well, any emotion can do that). From that angle I think this is a great film to spark a deep conversation with your pre-schooler.
My son is pretty emotionally mature, and asked some fairly good questions like "Why did Christopher Robin leave a note, and not just tell Pooh?", "Will Pooh be scared when Christopher Robin goes to school tomorrow?", which was a good way for me to segue into a great conversation. The fact that the return journey showed the animals just how different the landscape could look when you are scared was a fantastic opportunity to explain how it can feel to be scared. Things look bigger and more menacing, things that normally aren't scary can suddenly take on a life of their own. I'm sure every pre-schooler can relate to this, and it presents fear in a easily digestible form.
Bottom line, when/if you chose to watch this, sit down and engage with your little one when you do (actually, this is generally good advice, the TV is *not* a babysitter!). Be there to explain some of the scarier bits if they need reassurance. Keep pointing out that Christopher Robin is not lost, just at school, and that the fact that the animals misread the honey-covered note is critical (actually another really good message). Stay calm, don't freak out yourself, steer them away from the perception of danger, "Do you really think that noise is a Skullosaurus? Sounds more like a rumbly tumbly to me !". If you are worried about being caught unawares, watch it by yourself first to prep for the inevitable questions :)