I can't speak for Gutenberg books that are obtained from places other than Project Gutenberg, but when I get the plain vanilla e-texts (in ePub format) from the website itself, I find the quality and readability to be really good.
I download it directly via calibre (Project Gutenberg is one of the "Stores" in the "Get books" option on calibre), and then slap on whatever cover I feel like and then put it on my e-reader. Of course I can't speak for all specific books, I tend to mostly read from the top 100, but everything I've downloaded so far has been nice.
Many of the public domain classics are also really well done by many volunteers at mobileread too, and are available in epub format, and I've downloaded many books from here. I really get a thrill that I can get any public domain book for free. Before I knew about this I spent christmas money to get paperback copies of Jane Eyre, Emma, etc... then six months later I discover project gutenberg! Sigh. But I am very happy now.