Quake 3 is still the best shooter ever made. Quakelive = free Q3.
It's hard though. Also unforgiving with huge skill disparity. The main attraction is trying to beat the other player. There are players you could play for the next 5 years and probably never kill. So keep it in mind. But there's no game that is more satisfying that exists on the computer.
No good shooters have been released in a long time. CS:GO is the only decent one and that's only if you only like military style shooters... I personally hate them.