Seems as though most people believe Tousen to a weak character. Weaker than the rest of the captains because he lost to Kenpachi. First off Kenpachi is not a weak captain so there is no shame in losing to him. We also seen some of tousens strength when he was fighting the captains who are now vizards he looked pretty strong in those chapters.
We know that hes not affected by aizens sword because hes blind but if he was just some weak useless character Aizen would've just got rid of him. The fact that aizen and gin who are extremely powerful align themselves with him should show that he has alot of ability. Kenpachi could of been either, A. a bad matchup for Tousen considering he may be the only person who would overcome his bankai in that way or B. Tousen let Kenpachi win. I say this because it wouldve given him time to help Aizen complete his plan since if everyone thinks hes hurt or dead he goes unnoticed. He was also fine when he returned and we have no idea who healed him.