I think he’s one of the better apples amongst rotten ones. No matter how black & white people want to make it, every group has those. His “There are so few pleasures in life as fine as your company” is quite honestly one of the sweetest things I heard in the whole game, not to mention the fact that it came from someone who supposedly thinks I’m beneath him. He had no reason to give me such compliments since our business was done so it certainly felt nice. He also openly calls you a friend in the Embassy ("What is it, my friend?") and may even put his reputation on the line for you. In their own freaking party. If he was only concerned about gaining something, he wouldn’t do that. You make it clear from the start that you need him to make a scene just because you want to prank someone.
Ondolemar is the only Thalmor willing to address you as a friend and that, in my book, makes him special. Yeah, he might still be kind of a jerk but a jerk I actually like. It seems like he’s actually capable of learning and doesn’t have as one-set mind as the others. While he still goes along with the rotten Thalmor ideologies, there’s a difference in his overall attitude. He even compliments the Stormcloaks in his own way (“At least Ulfric’s men are willing to fight for their principles, barbaric as they may be.”) and actually appreciates how inquisitive you are. Rather than assuming some bullshit, he’s willing to indulge your curiosity. These things show he can think out of the box, unlike most of his colleagues.
I honestly think the game needed more characters like him because the very black & white way the Thalmor are portrayed is IMO a bad choice in a roleplaying game like Skyrim. Half of the time I can’t even take them seriously because it almost seems like the developers tried too hard to make them feel eeevil and thus only managed to give me an impression of some over-the-top bullies from a lame teen flick. One would think the Thalmor would be intelligent enough to have at least a basic level of diplomacy. Now they just bark arrogant and racist comments at everyone they meet. Even at other Altmers. How can a faction like that work? A more subtle and realistic approach would be way better. This black & white mentality isn’t good and it’s kind of sad how people go “No, Ondolemar is a Thalmor. He can’t be redeemed so how dare you like him!” People like the Dark Brotherhood even though they certainly aren’t heroes! They do evil shit all the time, yet many view them as “cool”.
So yes, because of these reasons I greatly enjoyed Ondolemar and I really appreciate his presence. He’s one of my favorite characters in the whole game because he actually managed to affect me in several ways. There was an actual development which went from initial dislike to like.