My first Kobo 1 had black lines appear on the screen after 11 months, but after some hassle Kobo replaced it free of charge. My Mom has my replacement Kobo-1 now and nothing has gone wrong with it yet.
I also had a Kobo Touch that I returned after 16 days (Two days past the time I could return in s
tore

), because it had a faulty Micro-USB port, and I've had the replacement for about 6 Months I think. But that wasn't a screen problem.
I also know 7 other people who have various models of Kobo with and nothing has gone wrong with theirs.
So I don't think it's really common.