Oooohh,I adore this! I like the use of detail and your colour choices. I really like when artists put in subtle things, like the darkening of the sky on Ulquiorra's side. It is obvious where you want the viewer's eye to go as these are the areas you paid the most attention to. You've done a great job at leading the eye.
That being said , there are a few areas which stand out as unfinished because they are prominant and lack detail. These would be the hole, the wing and the horns. Be careful where you cut detail. It may not stand out at a quick glance but when you do a great eye catching piece like this one, expect people to linger and really look.