It's been a while since I pulled out Knuth, but as I recall, it varies. (If nothing else, the problem sets range from easy to unsolved)
For the most part, the writing is very clear (he puts a lot of effort into it) but also quite concise (he covers a LOT of material!), so you may have to spend a bit of time working through exactly what's going on when you're reading about something unfamiliar. It's more of a reference than a textbook, so there's not a lot of handholding.