I kept looking for this post--I thought it was over in "Sources"--but couldn't find it...it's a good question, however and I couldn't let it go unanswered...
what are the pros and cons of roo compared to french calf?
The simple answer is that each has its strengths. But, generally, it is the customer who decides when we use kangaroo and when we use french calf. That said, a bootmaker needs some way of describing the various leathers he offers.
Kangaroo is one of the lightest weight leathers that is available--usually between ounce and a half and two ounces. It is also the strongest in terms of tensile strength--it won't tear as easily as other leathers of the same weight. While it doesn't have as smooth a grain surface as calf, it makes up into a beautiful lightweight boot.
French Calf is almost the gold standard when it comes to men's boots or shoes. Depending on finish, it has a very smooth and dense grain surface. it generally runs around four ounces in weight and many people like that combination of dense surface and substance around their foot. Although it's not common, a good "struck-through" calf has got to be a mainstay in every maker's shop.
Personally, I like kangaroo as well or a little better than French calf for my own boots but I make boots for myself out of both...repeatedly--having a pir of each at all times.
Hope that helps...