I'm a bit curious about how much of the manga does the Black Cat anime cover? I can't help but feel that it doesn't cover that much of it, with it only being 24 eps long and the manga being 20 volumes long. So can somebody help me out here with this?

It's a different story. Similar plot and most of the characters are the same to start with but it differs later on. I read the manga first so i might be a bit biased, but the anime dissapointed me at times. They changed some of my favourite character's personalities =( The anime cuts out quite a bit of the sidestories too.

I haven't seen the end of the anime but i hear it's a terrible ending xD
I might sound a bit negative but it's my second favourite manga of all time and the anime doesn't do it justice. Though im sure people watching the anime first will like it.

Everybody says that... Personally, I liked the ending, it was a lot of fun.
(vague spoilers ahead)
Although I've never tried the manga because when I made the transition from the anime's ultra-cute character designs to the manga, the lack of cute lost me (especially in Eve's case).

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It covers usualy about 70% of the manga, because many small things that the manga has the anime can't have(like ecchi parts, and expressions), most of the times the anime cuts the "not all that important" parts (that to me are really important) which make the manga funny and create a deep impression of the real thoughts of the char.....
About other mangas converted to animes:..... the way the sometimes they paint all thatHayate with BLUE HAIR!?(plz make it white! I can't detroy my image like this!)...it destroys the manga-like image of the story(with your own way of imaging the color of everything there)...... and there are times that the anime needs to "wait" the manga progress so they start creating fillers.....fillers @#$%¨&*...I hate them...like in Bleach that because of all those useless fillers the anime has now some stupids and useless extra char(my opinion, sry if I offended someone) this adding all makes me prefer reading then watching but some(few)times the anime is as great as the manga, read and watch, the important is enjoy.
well....I'm kinda little tiny bit down with animes later, so don't mind me ;)
sry if I talked too much and there is no need to read anymore(thank god for making me stop now! I can talk about absolutaly nothing for more than a few pages! ask my friends if you know any of them )

WHHHAAAAT? xD Eve is much cuter in the manga! ^^ (maybe not at the start)

I know what you mean aircrag, they skipped a few of the best jokes and scenes, you only get to know the characters personalities by their actions in the main storyline, (not really a spoiler unless you want to watch the anime first)

basically all of it but they just changed it up and added more to it cuz the manga didn't finish with the rest of the chrono numbers