Some of you may know of a design I did a year of so ago - Burr Bones, which was a 3 piece burr and was designed with the idea that as many voxels as possible were removed and still leave a non-trivial puzzle. It worked out better than I expected, and was made by Eric Fuller ( www.cubicdissection.com ) .
I have been playing with a six piece version since then, but it always seemed that there would be too much room for rotations giving a very unstable construction. This weekend, to try and get around these issues, I decided to change the sizes of the pieces, moving from a 4x4x10 piece to a 4x3x10, and making pieces that had more complex constructions. I have found a couple of interesting puzzles using Burr Tools, and below is a screen shot of one that is a high level, but uses 5 congruent pieces. It does have 6 solutions, but 5 of these are level 188.8.131.52.3 (39) and the one shown is 184.108.40.206.3 (41). Obviously, until these are made physically, the actual puzzle element and any shortcuts will not be obvious.
As I write, the highest level puzzle I have found is level 220.127.116.11.2 (48 moves!), but I am sure that adding a voxel or two will raise this :)