Eh. I have terrible taste, so I've eaten McDonalds around the US as well as in Paris and Bangalore.
Within the US, yeah, it's very consistent. I've not seen much variation, other than the one or two specially decorated locations and the menu is very consistent.
Internationally, the branding is very consistent, but the product isn't that consistent. The fries in Paris were very different (and not very good; my feeling is they probably used the same procedure but very different potatoes), but the burgers were pretty similar. In India, they don't serve burgers, but at least when I was there, they did have the delicious old school chicken nuggets that they replaced with 'all white-meat' bleh nuggets in the US. I didn't try any of their chicken sandwiches, because why when I could have the nuggets of my youth?
I understand there's significant regional differences in all the territories they operate in.
They purposely tailor items to the country they're in, and even add entirely new items. For example, the fries in Country X might be cooked with different ingredients or in a different manner, in an attempt to satisfy consumers in that demographic. You'd better believe they could serve US-style fries wherever they wanted, if they wanted. The reason they don't is not because they can't but because they don't want to.
It is remarkably consistent, tourists go to McD's very often when short on time because they know almost exactly what they’re getting, even though they might be a 10h flight from their home.