I guess I'm a little late in posting this but:
I think Megumi didn't have a great voice, but it had a lot of character. I think her voice made c-ute's songs more interesting in general. And I loved the song she, Miyabi, and Reina did (Suki ni Naccha Ikenai Hito).
Like a lot of people said, I think the photos were the last straw. I bet UFA told her she had to break up with her boyfriend and she refused. It seems like she HAD been thinking about quitting, but the break-off was still very sudden. Ah, young love......
Anyways, she did have a lot of potential and even though she's not one of my favorites I think she's a big loss to HP.