I use my Pistorius VN-J to join. It clamps up much like a stanley vice for a perfect corner, and I can see exactly where the v-nail is going to drive in.
I also have a AMP Miter-Mite I inherited from the last shop owner, but he didn't take care of it, nor his compressor. It's a hot mess I don't have time to figure out. I let him try to show my how to use it, just to watch him destroy a dozen frames--crushing the face of the frame, blowing vnails out the sides, air hissing out of every hose, never a tight join. It's supposed to be a good joiner, but I really don't know where to start... I certainly shouldn't listen to my predecessor, as he thinks 1/8" corner gaps are perfect.
Regardless, all I'm joining are generally small-medium softwoods. Many more difficult mouldings I'll just ask LJ to join for me, so that makes up some time...