Sounds like a great idea! My advice would be to start thinking of creative ways to present your reviews. What I mean by that is, there are a bunch of Movie Reviewers on Youtube, so you'll need to stand out from the crowd. Simply talking in front of a camera isn't going to cut it; you also need style. When I think of the Youtube reviewers I mostly watch (Jeremy Jahns and Chris Stuckman) I notice that they both have a good balance between substance and entertainment value. So my advice is to start thinking on the style and branding of your channel before you even do the first video. Do you get what I'm trying to say?
Rodolphe is very true. Youtube is heavily saturated with people doing a lot of things and people are constantly battling for the "viewers" time. The most important thing in life is your time and holding someone's attention and time is the key to success. I think it's a fantastic idea also, but you do need to consider how you want to present it, are you going to do all range of movies or just certain types? Doing a wide range does run the risk of being called out a lot on not knowing your stuff. Map out your end game before starting and make sure you stay true to yourself. Reach for the stars and realize any success will take at least a few years. If this industry was easy everyone would have done it.