Splicing is obviously better than knots. This works well for when you make up some permanent rigging for your hammock or other things, but there are many times that you want/need to just tie a knot.
Regarding amsteel and knots, I have heard two concerns. Some people claim that knots cannot hold due to the slippery nature of the amsteel line. Well, I recently received some amsteel, and after experimenting I'm pretty sure this isn't actually a problem if the right knots are used, even for a slide a grip knot. I suggest the klemheist over the prusik.
The other concern seems more likely. I have read that the nature of the material is such that it is weakened by knots much more than normal line. Is this true and why?
If amsteel is weakened too much to take knots, this seems like a major defect. I'd rather carry a heaver line that can be knotted. The weight difference probably isn't very much anyway.