Will definitely be picking up the new album. To me, the "must-have" albums are the run from Rust through Youthanasia, and including Hidden Treasures, and then a bit farther down the list, Risk (yup!). I also really liked The System Has Failed and Dystopia. I like all the other post-Rust albums, and was always glad to have new music, but rarely listen to them.
As for the pre-Rust albums, I have heard them many times through the years, but never really cared for them. Yes, I realize there are some "classics" in there. But as a whole, the songwriting, production, etc. is just a bit too "raw" for me. Never really suited my taste, even when I finally did start getting into thrash at the tail end of the '80s and into the '90s (yeah, I was a bit late).
As for the new songs, I like them. I suspect the new album will follow suit with the other fairly recent ones where I'll buy it, enjoy it for a bit, and then forget about it. And that's fine. I actually like the heaviness that their current downtuning brings to the newer songs, even though I think it kind of messes up the older songs, as others have mentioned. But no real desire to see them live. I saw them once in summer of '95 headlining a festival in Minnesota, and was glad I did--they were fantastic, but I don't really feel a need to see them again.