The day after i bought like 20 blank dvd's and i began downloading and burning like a ton of games. The big surprise was that no disc was good enough to play an entire game. Some fail to start, some fail to load in game and other are not read at all by the PS2.
After that, i bought other DVD's , 2x more expensive (still cheap, though) and i was very surprised that every game was playable.
TL;DR : Use quality DVD-R for backing up your PS2 games, the console is VERY picky regarding that ort of thing.