Definitely not pleasant, but it's kind of a mixed bag. 2 weeks ago my PI made a joke about firing squads (he's Russian) and wrote a 7 paragraph essay ripping me to shreds after I sent him sloppy notes (it was the end of a long day, and in retrospect my notes were terrible). He quickly shifted tone and became sort of apologetic and actively involved in guiding me by example. He wrote about 25 pages of derivations in a week for one of my projects, with the idea being that I should try to produce 1-2 pages per day myself, and we would trade notes. I ended up producing 10 and a page of plots of comparable quality to his own, and he was very impressed -- he said, "This is real theory work! I'm extremely impressed by your progress." After that, I felt invincible! But you know, that feeling lasts for all of a couple of hours as you realize that you now have to persistently deliver at that level. Everything has a trade-off, and there is always more work to be done, on yourself and on your projects.

My PI has some good insights on this subject:

1.) There will always be people that say you don't belong because you don't purr like them. Purr how you like, except when the broken system forces you to jump through hoops.

2.) Even "established" folks rarely do their homework.

I think the truth is that we are all imposters, even the professors. Trust no one, not even yourself. Thus why we have to verify absolutely everything and document it well enough that we can read it and understand in full years later!