The Russian Join is my best friend of joining yarn. It prevents me from having to weave in ends and I love not having to break out the tapestry needle as often!
I generally use this join when I'm working with the same yarn. You know, you come to the end of a skein and normally you'd end up with tails to weave in and whatnot, but not with the Russian Join! It's also great for when you find a knot in the middle of your skein (which has happened a lot to me lately for some reason...ugh).
It can be used for any yarn, but I like to control where my color changes happen so I would only use this join with two different color yarns if I was doing some scrap project where I didn't care where the change occurred.
When it's worked up, you can see a slight difference in where the joined yarn is versus the regular strand. However, I find this to be comparable to what it looks like once the ends have been woven in so it doesn't bother me in the slightest.
Oh, and I do recommend using a metal tapestry needle to do Russian Joins. You can use the plastic ones, but the eye of those are really big and find them just harder to work with for this.