Check out this new report from Environmental Defence Canada. The title sort of says it all: "Canada's Toxic Tar Sands: The Most Destructive Project On Earth" (PDF).
I found the title a bit overheated at first, but take a look before you decide. The claim may be debatable, but it's also not mere hyperbole: the tar sands oil extraction very well could be the most destructive project on earth. In fact, it's already yielding catastrophic results for human health, not to mention for a vast swath of North America's ecology. (In any case, I've had the privilege of working on climate policy a bit with one of the authors, Matt Price, and I can attest that he's a smart guy, not prone to exaggeration.)
I won't summarize the study here, but just point out that among the many problems with tar sands oil, is that it can only be extracted and processed with very large energy inputs (which means huge carbon emissions):