If you don't want to hear the "corporate stuff", read no further.
Just a little clarification first. Disney was "saved" essentially by Roy, Stanley Gold, and the Bass Brothers, with Roy being the driving force. It was that team that stopped the Michael Milkens of the world from breaking Disney into pieces and selling it off.
When it came time to put forth a management team, they wanted Frank Wells as the #1 guy, Eisner as the #2. Eisner was to provide the Hollywood clout it was felt Disney needed to move forward, but Wells was the business guy who was going to run the show. Eisner, however, would not accept the #2 position. Time was of the essence, and there weren't any other Hollywood bigwigs available at the time that were viewed as appealling options.
So, all agreed Eisner could be the head honcho, with Wells as President. However, Wells would not report to Eisner, but would report directly to the Board, just as Eisner did. (If you are really interested in how all of this went down, read "Storming the Magic Kingdom". Its not an anti-Eisner piece and primarily focuses on how the takeover/breakup was avoided and how Eisner/Wells came to be the new leaders.)
Disney was going to expand with or without Eisner, and with or without a breakup. The biggest reason it was a takeover target in the first place was not that it was necessarily performing all that badly. It was that it had a bunch of assets sitting around not doing anything. All that land in Florida was going to be worth a lot more developed than undeveloped, and Disney wasn't moving fast enough. The film library was just sitting there, for the most part.
So whoever bought WDW, for example, or whoever was brought in to run the company was going to start building stuff. Hotels and parks were going to happen. Eisner and Wells were not the only guys thinking this should happen. They were brought in to execute what everybody knew had to happen.
You can't credit Eisner, or even Wells, with the fact that things were built. The question is HOW did these things happen, and was it done optimally. (I'll leave that one alone for now.)
So was Eisner "good" or "bad"? Nothing is absolute. Certainly some of the things done under his watch turned out quite well. However, from many accounts (not just one book), he clashed with many of the other executives during those first 10 years, including Wells and Katzenberg. Many (though not all) of the creations we consider "winners" were actually done in spite of Eisner, not because of him.
What's also clear is that after the death of Wells, Eisner gained complete control of the company, and many say that things started going downhill at that point. Several years later, as the impact of Eisners complete control of strategic decisions was felt, the company's fianancial performance started stagnating. Still making money of course, but not performing as expected, and this has been reflected in the stock price since.
Really, the only question is did Eisner really change after Wells' death, or did we simply start seeing the impacts of what he would have always been capable of if in complete control? I think its the latter, but many think Eisner really did work well as part of the team at first. Either way, most agree he eventually overstayed his welcome, at least in part because he had so much control over the company and the board.