There are several put in and take out options. Generally, the rapids get tougher the further up you go, with Flight Simulator being the toughest and longest rapid near the beginning of the run. Confluence rapid is the only rapid above flight sim. It's very short and can get pretty tight. Just put in below anything you don't want to run, it's all great water.
I've run from about 450cfs to 1600cfs (Johnson Cr gauge). Not sure I'd go lower. And 1600 was pretty burly for me in my 12.5' Sotar Legend. I didn't run Flight Sim at that flow, but ran everything else. You can run top to bottom in a day, no problem, or do two upper runs, taking out at deadman (picnic area with outhouse).
I've mostly just seen small catarafts and kayaks. It's a technical run. I have seen one large cat and a couple rafts R2 it, but that's less common (but still awesome).
It's mostly class IV, with Flight Simulator at class V. There are a lot of rapids, but still some recovery room between most of them.
We usually run around 4th of July, but sometimes flows are too low by then. It's nice to go in there and run it for a day or two, then run the SF Salmon. If you do that, you don't need a lottery permit for the main if you just float out the last (boring) 20 miles in one day. I think you're supposed to get a special permit by calling the Krassel Ranger.
There are a bunch of EFSF shots here: WW Best of 09.m4v - YouTube
from about 1:15 to 2:15. That was at 450cfs.