This is something that has been bugging me too. Everyone is so quick to label Rey as "Mary Sue", it's like they dismiss all other points that don't show her as being perfect.
Believe it or not, there is a reason she may be good at using the force, and one we don't know about - Midichlorians; tiny microscopic life forms that allow force sensitive individuals to use the force. I'm not sure if anyone is aware, but you can just naturally have a high count of these Midichlorians living on you - which I'm guessing the higher the count, then supposedly the greater you can become at using the force. Since it's something that can happen naturally from birth, there isn't really anything to suggest Rey wasn't simply born with a high Midichlorian count.
Even if we don't go with that 'theory' as a way of suggesting why/how she was so good at using the force, we can still look at things she did that I'd say were less than "perfect". Remember when she jumps into the Millennium Falcon, and her and Finn are escaping from those ships? Well, Rey doesn't exactly 'shake' that ship off of her, like she told Finn she was going to do. Actually, it was Finn that shot it down; without Finn in that situation, nobody would have shot it down. Oh, and then there was the time when they first met Han and Chewie. Whose idea was it to close both of the doors on those bad guys? It was Rey's. What happened instead? She released those creatures (forgot the names). That was probably (well not probably, it was) the worst thing she could've done. Hardly perfect I'd say. Also, if she was meant to be so great, how comes all she does is go and hide near the end until she meets back up with Finn and Solo? It's not like she took on any Stormtroopers or anything, not like they do in all of the other Star Wars movies.
You can't just say Rey was such a flawlessly perfect character, not when you've got Kylo Ren who does similar amazing feats, as to which nobody says "Oh man, wasn't he just a little too good in the movie?". It works both ways, Rey and Kylo are only just beginning to learn what they can achieve. Nobody questions how Kylo - not even a Sith Lord, yet - can do half the things he's doing, which to me doesn't sit right. The criticism should work both ways, because it seemed pretty equal to me.