Kind of a necropost but I'm gonna chime in with some real world data. I used a homemade tumbler after seeing my buddies 1600 Tom's tumbler. I used it for years until I picked up a cheap used 1900 model.
1) Yes, it does knock more kief off than hand trimming. Not an extreme amount, and it all ends up in your extract. I view these machines as useful if you have more weight than you can handle hand trimming.
2) you can control how much kief comes off. How long you dry it and to what RH you dry it to effects how much kief will come off. If you only dry/cure to where the bud is still a bit sticky but the leaves are sorta brittle you will keep more kief, but there will be some leaves still on there.
3) You still need to finish hand trim. It goes by REALLY fast compared to hand trimming, you are more or less just doing a quality check and trimming the few errant leaves that did not come off.
Dryer bud=Less touch up, more kief loss. Sticky bud = more touch up, less kief loss. Once again, we aren't talking a lot of kief loss, and its right below in a bucket. you could sprinkle it back on the bud if you felt like it.
So, if you want High Times quality photo finish buds cause that's what gets your rocks off, don't buy this.
If your hands hurt like hell and you still have pounds to go and it's taking forever, get one yesterday.