Wow, lots of fun to chime in. I must think I know something (grin). Probably not but here goes: I agree with most of the pervious comments. The Grand is big but mostly friendly and pool drop so you can set up and not get pushed where you don't want to be. The Main Salmon at high water (6+ feet) is much pushier and like all Idaho Rivers, very fast. Cat at high water (40+) is bigger than most of what you'll see in the Grand at typical flows. Westwater in the teens can be more difficult - - it's "roilier", pushier and more unpredictable due to the hydraulics. The holes to miss in the Grand are mostly obvious and scoutable, but you don't need to scout everything either. I ran my first Grand in the 17-20K range and we only scouted House Rock, Granite, Crystal, Upset and Lava. Scouting Upset didn't help we still had a flip. (Not me, but here's what I took away from that one - - always remember to ferry angle your boat to get where you want to go - - simply rowing upstream against the current does no good if you're not changing your line!) Ran all the rest by "Read And Run." At Lava you just run big coz there's no other way, left or right (grin). Good idea to look at Crystal and see what the main hole is doing. Some levels it's runnable, others not. There are left side runs at Crystal and Upset. We've had flips at the following: House Rock, 24 1/2 Mile, Tanner, Spector, Upset, Lava, 209 Mile (best to just miss this one, stinker of a hole, nothing to prove, just go around it) and swims in a couple others. Probably the only real important thing to remember is DON'T GO LEFT AT BEDROCK. It's survivable but can be a MAJOR headache. Flips happen. Rig for em and GO BIG! Most of all just enjoy being down there. It's AWESOME!
PS YouTube is a great place to study most of major rapids nowadays. Especially helpful to figure out the best line at Lava both left and right. Vaya con dios.