I've got the 10x and 20x versions of this. The 20x is unusable unless looking at transparent/translucent backlit samples. The 10x is extremely handy- easy to get enough light on the sample while getting a close look, and shows you a lot of detail. I haven't tried the 15x personally, so hopefully someone else can chime in, but do not get the 20x.
These things are invincible btw.