As I wrote before, JJ and the other writers went out of their way to make the new characters as interesting as possible. I really couldn't find one boring or stupid character out of the new batch. Even the smaller villains have something interesting traits right down to some of the Stormtroopers. I think these new characters fit into the newer environment with ease. Remember this is set 30 odd years after ep6. So, the universe they live in is much different.
The old cast is just great in their old roles. Each old character gets a “Hey remember me moment.” I love seeing these old friends I grew up watching finally come back. Plus, there are nice interactions between the old and new cast members. And, I love every minute of these interactions.
BB-8: I truly love this character a lot. He is one of my favorite characters from the new cast. He has as much personally as R2-D2, but completely different. This robot has a lot of emotion, and he really shines in the first two acts of the movie. BB-8 has a great moment where he sees his master again and rolls frantically toward him as a dog does when he meets his owner after period of absence. He's right up there with RD-D2 as far as robot characters are concerned. This robot is great.
Rey: For all purposes, Rey is the main hero in the same manner as Luke in many ways. Yet, she lives a slightly rougher life than Luke because she is a scavenger and deals with people that are shady. I think Daisy does amazing job and she is going to be a great lead actress for the rest of the series. I also like that she is a female lead that can take care of herself. Daisy Ridley is a wonderful addition to the franchise and carries herself among the old cast. That was very good casting, that's a plus given the casting in the prequels. She is the new Luke Skywalker.
Finn: Finn is played by John Boyega is an bright eyed former Stormtrooper. He is literally new to the rest of the world when he defects. He grew up only knowing the First Order and we get to see him view things through free eyes and it is fun. Speaking of fun, I love the way Rey and Finn just think on the fly when trying to solve problems. They're a bit looser with things than the old guard.
Kylo Ren: He's the main bad guy front and center, but he's working for a bigger bad guy. Kylo appears as the total badass villain that knows how to use the force. He has a new bag of force tricks and they're cool. However, Ren is a highly flawed character. He's a spoiled brat that has fits of rage when something doesn't go his way. He is a wannabe Sith lord, but doesn't have the skill like Maul or Vader. He's all power and no true skill, and he wants to be the total badass by dressing and sounding like the Sith guys. He is not Vader, but he has the potential to become like his “hero”.
He's still a compelling villain with an interesting past and a tragic one at that. I won't give away history behind this guy, but you will like it. I know a lot o people had a problem with him being a somewhat weaker villain than Vader, but I like that because he will become a total badass guy in the next two movies.
Poe Dameron : He's part Luke (ep 4) and part Han Solo in his attitude. He has swagger, he's a good pilot and very good sniper too. He truly fits into the franchise. He's relationship with Finn is very cool and almost mirrors Han and Luke's relationship. They're basically brothers. I also love the relationship he has with his droid BB8. It feels like a loving relationship. You can see how much they care for each other just in that opening scene.
Han Solo: Let's just say that he is very important character to the overall plot of Force Awakens. Han is not the same character he was at the end of Return of the Jedi. He's basically reset himself back to his smuggler days, but without the Falcon. Ford is really energized in this movie compared to his role in Return. Ford is truly remarkable reprising his role and he has the biggest part from the old cast of characters. I loved his relationship with both Rey and Finn. He's almost a father figure to these too. Ford is having a lot of fun.
Princess Leia: She is the driving force for the plot, but you're not going to see her for the first two acts. Her character is the reason the plot moves forward. Carrie is a bit shaky in a few scenes and you can tell she's lived a hard life. However, she looks healthy and good.