It's funny, because I'm of entirely the opposite opinion now, but when I was young I actually couldn't get into the Zahn books that much.
Looking back, I think I didn't really understand much about character and narrative structure, and why they were important to making a story hold together. I was mostly interested in the sci-fi action and technology and things like that. I had been deeply upset by ESB the first time I saw it, because it seemed too dark and disturbing after the fun of the first one, and for a while I actually thought RotJ was the best of all three! It is perhaps no surprise that I was at first drawn to things like the ridiculous Kevin J. Anderson books, while finding Zahn's work to be too slow moving and dull, and I only ever skimmed over them and never read them all the way through.
I didn't try to read them again for a long while--but disgusted with the New Jedi Order series, I decided to give the Thrawn trilogy another shot. This time I was able to see all their merits that had previously gone over my head. I distinctly remember being astonished and delighted by the excellence of the characterisation, the momentum and intricate twists of the plot, and of course Zahn's unparalleled brilliance with creating science fiction technologies and putting them to all sorts of exciting uses, which somehow as a child I had missed completely despite it being a great interest. And having outgrown my need to see the good guys always outmatch their foes, the notion of something like the ysalamiri that could remove Luke's advantage lent certain scenes that much more excitement.
(For the record, ysalamiri and midichlorians are not conceptually similar at all. Zahn himself has said outright that he greatly dislikes the midichlorian idea, preferring a more mystical and mysterious view of the Force, and the ysalamiri in this regard are not so different from the various magical creatures that appear in fantasy stories.)
I think the description of Zahn as someone who really gets Star Wars is spot on. He has a definite understanding of what it's all about and what makes it work, and his stories reflect that in a way that very few others have been able to achieve. His additions to the world have a unique sensibility all their own that blends deftly into the original material, without changing it or trying to foist some kind of contradictory alternate intrepretation. Best of all for those who despise the prequels, the Thrawn trilogy draws only from the original films and nothing else, and even contains hints of a vastly different clone wars backstory--the Old Republic military (which had always existed) engaged in fierce battles with unknown "clonemasters" trying to take over the galaxy, as well as several renegade Dark Jedi with their own agendas. Unfortunately, he wasn't allowed to put in much detail about these ideas, but what there is sounds infinitely more interesting than anything Lucas has ever done lately!
I'm very glad I gave Zahn a second chance, because his works provided all I could have ever dreamed of in Star Wars sequels. The trilogy is undisputably the best, but the Hand of Thrawn set is very good also, particularly for the scenes on Nirauan with Luke and Mara Jade. A few years ago I managed to persuade my dad to read the Zahn books, and while initially sceptical he ended up enjoying them a great deal as well.