I've made and used them.
You get used to what is to modern eyes a 'poor' light.
It's quite surprising how well we can actually see with them once your eyesight settles to it.
I used to think that there was no way one could accurately do stuff like fine stitching, or good leather or woodworking, but the reality is that you can.
It's a smokey light though, and gross though it may be, you only realise just how sooty is it when you blow your nose
but even your eyes end up smutty with it too.
On the whole, yeah fun to know how, fun to work with and know that you can, but ehm, well we know that soot and smoke is not good for lungs, so if I don't have to, I'll find another way.
Incidentally, the pine splints work just as well, and though sooty, no where near as bad I found, and the smell isn't so clingy. The rush lights, even made of good suet, smell weirdly waxy, the pine splints smell of resinous timber, and I'm pretty sure that what they give off is at least mildly insecticidal.
M