I start reading Twain's Autobiography at any page and don't want to stop, for the sheer voluptuous pleasure of the prose.
I am pretty sure that's also how Twain felt about the Autobiography.
I'm struck, first of all, by Ebert's observation that you can just drop in anywhere in the Autobiography. That's true. Twain rambles on and on about what he believes to be his own clever innovation, viz. to dictate whatever the hell he feels for a few hours a day, on any subject, past, present, or future, and call it an autobiography. So the Autobiography is arranged chronologically by date of dictation, not by the sequence of events in Twain's life.
And, second of all, it's interesting that Ebert's in love with the prose, because this is Twain's prose at its loosest. As Ebert elsewhere tweets, it's "MUCH longer than the 'Autobiography' we know!"
The editors at the MTP work very hard, but their aim is not to improve the Autobiography, but rather to retrieve the most authoritative possible text by looking at all the variant drafts, figuring out which notations are in Twain's hand and which were made by Albert Bigelow Paine or Clara (Clemens) Gabrilowisch, or some random editor at the North American Review. That means that this edition is pretty unabridged. There's no more of ABP's and CG's judicious (or cautious) cutting. It's full-on Twain, which isn't, stylistically speaking, always a good thing. Perhaps he would have written a shorter autobiography, but he ran out of time.
One of Twain's favorite syntactic forms was, "X was ADJECTIVE and ADJECTIVE and ADJECTIVE and ADJECTIVE." He really does get lost in his own descriptions, and he can't resist tacking just one more modifier on there.
He couldn't have a more sympathetic reader than Roger Ebert, it seems. I hope Ebert writes more about Twain; he doesn't have Twain's bitterness, but in a way they're kindred souls.
Also, I suspect that Twain, like Ebert, would have loved Twitter.