Drew Karpyshyn's opinion(of Darth Bane) about Darth Sidious
Interviewer: "What would Darth Bane think of Darth Sidious and his stewardship of the Sith Order?"
Drew Karpyshyn: "My completely unofficial opinion is that he’d be disappointed in Sidious. I got the feeling the Emperor wasn’t really interested in finding someone strong enough to take his place; he was more interested in ruling forever. He also violated the Rule of Two several times – he had Darth Maul working for him while Dooku was his apprentice, and when Vader was his apprentice his focus was on using Vader to recruit Luke. Add in some of the extended universe information and you find out that Sidious had all sorts of people he was grooming and training in the dark side – he twisted the Rule of Two into something Bane wouldn’t even recognize. If you ask Bane, he’d probably say that’s why the Emperor lost."