Excellent
I really enjoyed playing this game, and I really like the concept. It was very well thought out and executed.
The graphics were good (though there is always room for improvement), the sound was also good (giving the user control over the sound is definatley a good idea - personally I prefer playing my own music when playing games) and the gameplay was challenging. I also enjoyed the attention to detail in the game - allowing the player to change the colour of their virus, and actually naming the enemies to name but two.
My only complaint is that the points needed to mutate were fairly few and far between - if there was a way of getting more per level, for example killing a certain number of bloodcells in a time limit with a bonus evo point per cell killed, or something similar.
Otherwise, and overall, I found this a very enjoyable game, with a lot of playability. Good job!