Of all the books I've read, I think Shadow Heir had the most unsatisfying series ending. I adored Dark Swan and I recommended it to everyone...until I finished the last book. I haven't recommended it to anyone since then.
I can't say too much about the conclusion because it wouldn't make any sense unless I used some pretty big spoilers. So I'm not going into any details. All I can say is that when I invest so much of my time and emotions into characters that I've come to love, I get upset when I feel like they have been wronged. It's also upsetting to me when a series ends and I'm pissed off at one of my favorite characters. It's over and I'll never know if the problem will be resolved and I just have to deal with it.
Is there an ending you would like to change?