Thrawn was by far the better villain. He was well thought out, and really worked. In many ways, was a perfect replacment villain for Darth Vader and the Emperor.
I am actually one of those guys who really liked Shadows of the Empire. It is a flawed book, but I liked it. And I even really liked Dash Rendar, I thought he was much more than just a Han Solo stand in that most people see him as. To me, he really worked as a character and added something to the story.
Xizor on the other hand, I thought it was a cool idea going with the whole intergalactic crime organization. The character in general, really worked. The thing with Palpatine trusting him so much, really made the Emperor out to be a total dunce. The idea that Xizor was in the throne room listening to Palps. "Son of Skywalker" conversation really bugs me, because that just wouldn't happen. Also Xizors obession with trying to humilate Vader in the eyes of the Emperor and lose his son at the same time, was a bit weak.
As for whoever said Xizor was a waste of a character whose only ambition was to be the Emperor's lacky, that was not it at all, you must have really missed a lot when reading the book. Xizor did want to be the Emperor's right hand man, but only as a necessary step in making his way up the ladder. Xizor was already head of the galaxies largest crim syndicate, he was a very powerful person, he considered himself third most powerful in the galaxy, right after Vader. His goal was to be first. He didn't care for the Emperor, he wanted to use him. Though, as shown by the way Vader totally owns him, he had no freakin idea what kind of powers he was messing with.
At the end of the book, the Emperor hits that he let Xizor in to his confidence in order to test Vader, and that he never really trusted Xizor. Fine, that sound okay, but still, having him in the throne room listening to extemely important conversations??? Weakest bit of the book to me.
As for, "Part of the book is Leia fighting against some kind of pheremones Xizor lets off that makes her want to make out with him."
LOL, that is the funniest way I could imagine that being explined. It was part of the book, as in, a few pages and a minor part of the story. Leia has a meeting with Xizor, and he releases pheremones to make her attracted to him, she realizes something weird is going on, and fights it off and resists, and he gets pissed off and locks her in her quarters. Also, I am pretty sure it wasn't making her "want to make out with him" for much as something else. Still don't see why people make such a big deal about that part of the story, it wasn't that unbelievable, essentially she was being drugged.
I also don't see what is so wrong with the story making everything from ROTJ "come together in a nice little bow". Leia had to get the detonator and the bounty hunter armor from someplace, what is so wrong about the story explaining where she got it from. Chewie has his hair cut differently in SOTE in order to resemble a famous Wookie bounty hunter, which explains his different hair style in ROTJ. Sure, this stuff doesn't need to be explained, but I don't see a problem with it doing so. To me, the least crimes of the prequels are far worse than the harshed crimes of SOTE. I'd take SOTE over any of the prequels any day of the week.