After hearing someone mention that we don't know anything about Sai (so untrue), I thought we should discuss his capabilities.
Well, he caught Yamato. He doesn't rush into a fight. Doesn't seem like he has any emotions - though he isn't as dead as Shino. On that last note, he seems to be one of the few characters that doesn't have to take a power downgrade to radiate his offense and defense in very direction (like Naruto's throw away clones). He seems well trained. He isn't afraid of simple threats like how Sakura, Shino, Kakashi, Yamato and others have all scared Naruto (manga only here, too).
Some weaknesses. At full strength, he can't see Sasuke at top speed. We haven't seen Sai take a hit (Sakura doesn't count; gennins have always taken rock crushing blows). He talks too much. He doesn't know not to look at genjutsu artists.
I'm sure I'm missing other points about his weaknesses. The list for bad points isn't long but they are important against some opponents.