I've played 1 2 4 5 and 6 and completed all but 1 and 4. They're just not very good. Very basic in every way. 6, by far the best of the bunch, just has very flat, bland characters and dialogue, a cliche plot, boring 12-bar-blues music, too much dungeon crawling and level grinding, basically just nothing going on that drew me in to it. Maybe I was just too old when I played it and noticed all the problems.
