Biohazard HD by Meyrink is probably first theme skin that I would recommend for serious playing. This skin is clear and that "biohazard" theme fits in perfectly. Although hitsounds are copied from various skin, that failsound is absolutely amazing, Meyrink is genious.
Cursor doesn't fit in with this red color theme, but I think if red cursor wouldn't be really visible, so I think that cursor is fine.
Spinner works really well, clean background makes it clear to spin.
+ Sounds work well
+ Additional content
- Cursor not in theme
- Sounds copied (doesn't really matter)
This skin is still on development so download link here might give you old version of skin, you will find the newest version at Osu! forum thread