I don't know what all of them are really like, however:
I once heard Mutya sing live, when she was on Big Brother, and she was VERY good. I was really impressed. The HMs had some sort of singing competition, and there were a few other singers on there who did pretty badly, but Mutya was leagues better than the rest of them.
Also, I saw a youtube video of Amelle and Tinchy doing an 'a capella' version of Never Leave You, and I was very impressed by Amelle's singing there too.
And I know Jade is a decent singer.