Duality

I really do like the way you've painted this, very smooth and great use of colours.
The way you've painted the knife is fantastic, the blade especially, very realistic.
It's also a fantastic concept that really did make me think! <img src="e.deviantart.com/emoticons/g/g…" width="17" height="15" alt="" title="Giggle" />
The thing that I really would suggest is finding a consistent light source - there's parts of this of this image that suggest the main illumination is coming from the left, parts that suggest it's coming from the right and parts that make me think it's coming from straight on!
I might also try adding a couple more highlights to the hair, to make it seem as shiny and full of life as the figure, and possibly thinning the neck a little where it meets the shoulders - the effect it gives at the moment is that she's straining heavily, which really looks a little odd with her expression.
Some people might suggest that you blend your colours a little in the facial area, but I like the impressionistic effect that this gives.
Also, feel I should comment on the dress - absolutely love this! You've painted it really nicely, love the shadows. <img src="e.deviantart.com/emoticons/b/b…" width="15" height="15" alt="" title=" (Big Grin)" />

You asked for critique so I'm offering you some - feel free to just disregard it if you don't like it - this piece is fantastic as it is.