Leonardo DiCaprio is generally good in whatever role he performs, which is why he's paid so much. He is believable as the anti-hero Dom Cobb. No complaints there.
However, his supporting cast absolutely steal the show.
I tend to trust any movie with Cillian Murphy and/or Pete Postlethwaite in it. They are both in Inception and they did nothing to dent my trust in them as actors.
Tom Hardy has a new fan. I vaguely remembered him from Flood, but he didn't stand out there. He did in Inception. I found my attention drawn to him in every scene that he was in.
Michael Caine played Michael Caine, as he always does; while everyone else made me care about what happened to them.
If there was a failing in plot and narrative, it was that Inception failed the Bechdel Test hands down.
This examines the role of women in movies with a very straight forward criteria:
It has to have at least two [named] women in it...
Yes, Ariadne and Mal are both female and both named. They have very prominent roles within the plot. Arguably, they have the two most vital, polar opposite roles of all in the movie. So far so good.
... Who talk to each other...
Yes, they do. In two separate scenes, they have a conversation.
... About something besides a man.
No! Every bit of dialogue they have together revolves around Dom Cobb! This was disappointing, though understandable within the storyline.