I'm with the "give it a few days" camp. Really, I do think it looks nice on you. But, I also know what a bummer it is to glance in the mirror and get a shock because you don't look like yourself.
In it's natural state, my hair is medium to dark brown with a whole lot of grey mixed in. My hair is curly and dry, so to save on damage, I only get my whole head dyed about every 8 months. This means that for 8 months, my hair fades lighter and lighter and just the roots get touched up. Then, bam, it is dark again. And, I dislike it for the first week because it just seems SO dark. I startle myself in the mirror.
Honestly, I really do think it looks good on you! Give yourself a week or two and then reassess. I also second a pp about make-up (if you are a make-up wearer). I wear minimal make-up usually, but I find that my darker hair seems more balanced during that first week after a dye job if I wear a bit more lipstick and such.