I want to thank Kulyok and other people who worked on the mod for their mod and to submit my experiences with it.
First, I decided to try this mod based on my experiences with BG1NPC Project and my playthroughs I did with Xan in my BG1 games. I played through both romance paths in the BG1 and also tried the friendship path. They were all solidly done, perhaps not outstanding (I liked Coran's and Dynaheir's romances more), but very enjoyable nevertheless.
I first tried to play with Xan in BG2 with a PC who was a female elf, choosing a 'continuation' romance to simulate the continuity between my previous playthrough and that one. It was, again, solidly done, but I found it difficult to enjoy the game because the romance was too melodramatic and - ah - shoujo-ish, for the lack of better word. Judging from the overwhelmingly positive reviews, I conclude that, perhaps, it is just a matter of the mismatch of my gender and the intended target auditory - I am a young man, and the mod was mostly praised by female auditory. Nothing wrong with it, but romancing Xan greatly grated on my nerves. So, I shelved the game for a year and didn't employ Xan until now.
However, I tried Xan again with the friendship path and the quality thereof just blew my mind. In this aspect, I found the mod to be outstandingly well done.
1) Integration.
First of all, Xan is seamlessly integrated into the game, taking as much spotlight as needed. My opinion is that he escaped both the caveat of stealing a game for himself and the caveat of being just a tag-along extra. He felt in place with the events of the BG2 just as much as, say, Imoen, he mattered, he was an actor (=figure) in the game world and his profession as the defender of the elvendom tied nicely with the quest for stopping Irenicus. The few quests he had were both interesting from a gamist point of view and developed his character and the world around him.
2) Balance
Xan was nicely balanced as an NPC, he had adequate stats and got adequate items. I used him as an fighter-mage, and he was one of the strongest members of the party. I guess he could be useful even in the most tactics-heavy runs of the game without being overpowered.
3) Character
What I really liked was how Kulyok's developed him from the bare-bones character in the original BG1 into the complex, nuanced person. In the BG1NPC Project Xan was interesting, in the BG2 he really starts to shine. His relationship with the PC feels organic and developing over time: he starts in the BG1 as a rather cold and introverted person and progresses to the point where he empathizes with <CHARNAME>'s stife and troubles. The high point of the mod, I think, was Xan's declaration at the end of the SoA that he is ready to die for the Gorion's Ward.
I liked Xan's banters, interjections and PIDs. Some of the PIDs you could initiate were, frankly, amusing. Other NPCs were kept in character, I feel.
4) Voicing
SIsterVigilante's voice sounds a tad strange after the Jeff Bennett's work in the BG1, but over time I acquired a liking to it. It was well done and didn't seem to be out of the professionally done voices of the native BG2 NPCs. In fact, I think that SisterVigilante's work here is better than in his Angelo mod.
All in all, if anybody is reading this forum trying to discern whether to try Xan in a BG2 game, I think he will be a great addition to your party even if you don't intend to try to romance him.