I wouldn't say I find Harmony annoying. She's definitely much more interesting and funny than annoying!
And I do admire Mercedes McNab's acting. I think it's harder than one might imagine to play a character like Harmony and be so good at it. I love this dialogue from S5 "Crush":
Harmony: Who is - oh, wait. I get it. Our little sex game was just the beginning. Now you've gone and picked up some cheap queen of the damned to dress up like your precious Drood-zilla.
Spike: Harm.
Harmony: You'd better not be thinking what I think you're thinking. 'Cause my answer is the same as always. No threesomes unless it's boy, boy, (indicates herself) girl. Or Charlize Theron.
Spike: Harm, you moron, this IS Drusilla.
BTW I think Juliet Landau aka Drusilla was fine too.