If your looking purely on image quality, it may or may not be worth your upgrade, but as the overall camera goes, it's almost in a different class compared to the 5d2, which I'm intrigued with.
Very true. IQ aside, the dual card slots, weather sealing, and vastly improved AF and FPS make is a much more versatile tool, and much better suited for professional use. IMHO, the 5D line has been an experiment in progress for Canon. If you think back to 2005 when the 5DC was announced, it was a game changer. Full-frame bodies were the holy grail of DSLRs back then, and the only option up until that point were $7-$8K 1-series bodies. Full-frame bodies were big bucks, bulky and simply inaccessible for most photogs. No one dreamed the day would ever come when you could buy a small-form, full-frame body for $3,300. It was a stunning move to say the least.
That said, I think Canon has always viewed the 5D line as a consumer grade product even up until the MKII, and the build quality and feature set has reflected this sentiment. Perhaps Canon underestimated how popular the 5D line would prove to be, but I think the number of 5Ds used professionally far exceeded its expectations. As such, Canon has integrated many pro-grade features into the 5DIII that people have been demanding for a long time.
Sure, Nikon has developed an amazing camera in the D800 for $500 less - and I'm very grateful that competition like this puts pressure on Canon to up its game - but that doesn't change the fact that the 5DIII has many improved features over it's predecessor that I'll put to great use on every single shoot.