Whats the compression like with the DVCPro HD format?

Am I going to have trouble with compression artifacts when I start doing key's and colour corection?

Well, the question is: compared to what?

DVCPRO-HD is comparable to HDCAM; it's way better than HDV, and not in the same class as HDCAM SR.

DVCPRO-HD is 4:2:2, so you'll get much better keying with it than you will with any flavor of HDV. However, it's not as noise-free as DVCPRO50; there is more mosquito noise in DVCPRO-HD than there is in DVCPRO50.

So no, it's not perfect. I do believe it's the best format available to the budget HD shooter right now, but I would suspect that the JPEG-2000 of the Infinity camera would probably be cleaner etc.