hes alright not that special he was a anbu and arent anbu suppose to be jonin level hes high chunin to me
I think you are underrating Sai here. Remember this guy is pretty fast and stealthy. He can also create any animals by using ink. This makes him pretty versatile such as being able to fly and create all sorts of beasts for defense and attack.
His intelligence is on par with Sasuke's according to the data book or at least that is what I remember. Remember he held his own against Choji, Shikamaru and Naruto so he is definitely jounin level. I'd personally put him at mid jounin level. Also Danzo trusted Sai into killing Sasuke which means that Danzo thought that Sai would be a formidable opponent.
During the war, he was able to blitz Deidara, an akatsuki agent.
OT: Well I think Sai is probably heading towards Naruto with the rest of Konoha 11 to fight Tobi.
As for him being a good replacement for Sasuke, umm no. Sasuke is much cooler and overall a more interesting character than Sai. Sai is sort of bland and isn't that important.