Counter-Strike: Source... arguably the best FPS period.
The graphics in this game are amazing! All the particle, smoke and water effects are extremely well done. There is a reason the Source graphics engine is so popular and it shows in this game. If you don't have an up-to-date computer you may have to sacrifice some visuals for performance, but the Source engine scales quite well, so as long as you aren't years behind you should be able to find settings that work.
A heads up to any parents up there, there is a good amount of blood in this game and obviously it is centered around shooting other people, so you may want to keep that in mind if you are purchasing for a kid.
Now we move on to the actual gameplay. I already touched on the one life per round aspect of this game, but as a new player you should be prepared to die...... a lot! You will die over and over and you will never kill anyone. That is a fact, and if you are going to buy this game it would serve you well to just accept that right now. Having played First Person Shooters in the past will help a bit, but you will still be nothing but a bullet absorber when you first start out. The learning curve on this game is a bit steep, and there are many people out there who have been playing this game for years. They have the aiming system down to a science and know every level like the back of their hand. There is also a little weapon called the AWP, which is a sniper rifle you will get to know very well. Mainly by the amount of times you are killed by it. This is a one shot one kill weapon and there are some players out there who simply don't miss with it. Suffice to say it can be quit frustrating when you are constantly killed from 200 yards while you barely get a chance to even see who shot you.
The point of all this is that it will require a bit of patience to learn this game and get to a skill level where you can be competitive. There is a neat little option where you can start your own server and fill it with "bots", which are computer controlled characters, and use this as practice. You can even set the difficulty level of the bots. While no AI can compete with human intelligence this is a good way to get better at the game and learn the levels if you are getting frustrated on real servers.
Overall, it's a great game that is being updated constantly with new levels and features. If you can get past the learning curve you will be rewarded with a satisfying online shooter that you will be playing for years to come. If you are easily frustrated and do not want to put in the time it takes to get good at this game you may want to pass on this and try something else.