Terry Pratchett could not hold me for more than a page or two, I suppose I am pickier than you. Likely the pickiest of types...
None of the authors I listed (Asimov, Lem, Clarke, Strugatski) has/have produced a piece which can be called "terribly written", not sure to which "classics" you refer (I can think of many other authors I have read and enjoyed but not of many I'd put in the "classics" bin).
I read a lot less novels these days, far from feeling restless if I have nothing to read (this started somewhere between 40 and 50). I have tried this and that and well, not many could keep me reading. I have re-read plenty OTOH.... A few years ago I stumbled across "The Sky Lords" by John Brosnan, it was good enough (I read the Bulgarian translation). Pretty good entertainment, will not bore you.
I wonder how the classics in the crime genre come across for people living in the US, I mean the brilliant ones, like Chandler, Hammett.
Dimiter