I got MMOCW and MMXOCW today, and even though there isn't a whole lot of plot-relevant info, I do like all the behind-the-scenes snippits throughout (Inafune has this, "I tried to focus more on my duties as Producer, but told everyone not to touch Zero" attitude during X2-X4). Bearing in mind that in Japan these are the same book, the main "series developer shout-out to the fans" section is in MMOCW, and is only two pages long. Both books have a single page of brief character descriptions.
Here and there are a few things that are nice to have clarification on. For instance, SB20's info from the old sourcebooks mentions Iris falling for Zero and Zero saving her life in the same sentence, leaving a possible implication that the two happened at once. Even though you can gather opposing implications from the X1-6 OST art (and even X4's own flashbacks), MMXOCW clarifies, stating directly that Iris developed feelings for Zero during the Erasure incidents.
Speaking of Iris, we also get a bit of Sensei's personal opinion on the concept of Reploid aging, as he mentions that he was deliberately trying to make Iris look younger for Xtreme2. That includes her being less "physically developed".
Curious info about Berkana, as well. She's a member of a Reploid research & development team, and has allegedly created her own Reploids, but none were publicly revealed. And remember that unseen lion from Xtreme2 art? Despite the likeness to Sigma's final body, the big cat is actually supposed to belong to Gareth. They just never figured out a way for him to appear in-game.
Oh yeah, and X? He wasn't Inafune's second-design for the new MegaMan; Inafune in fact did not design him at all. Credit for that goes to Hayato Kaji. Inafune did, however, make the call for X to be the new MegaMan and Zero to be support. Although he did envision Zero as a playable character, he figured his boss would be less critical of Zero as a sub-character (being red I guess got you stuck in that spot back then).