90% off Serious Sam Complete Pack

Just wait for Spring/Summer Sale. Serious Sam titles get a huge discount periodically.

To OP: I'd say, if you have the money, and would play it now, buy it. You won't miss anything, since they usually do a huge discount (80-90%) per quarter year. It's fun, but nothing brain chsllenging: You arrive to an arena, then monsters come to kill you. Not just 5 or 10, but hundreds! And you are repeating this level after level, sometimes with a boss, but "boss" means, that you must shoot it with all you've got. I liked it as a child, but if you are new, play it as one level per day, cause that way, it hopefully won't be that monotone