Well, RedRolling is a former trucker, and I am not. So I would trust his judgment over mine.
Having said that, I have taken 89 north to south, towing my small trailer, and it was not a problem at all. I did have to downshift several times, but I do that on many downgrades. And my wife said the scenery was great. I was looking at the road, so I can't comment. ;)
There is one hike at the crest that you do not want to miss -- it is called "Bumpa$$ Heck." (The names have been changed slightly to avoid the anti-obscenity software.).
Bumpa$$ was the name of the poor guy who discovered it by getting burned and dying. Here is a blog post that may be useful -- scroll down to the part on Lassen:
Lassen post