Serious Sam HD: First Encounter is a great remake to the original First Encounter.
The first pro in this game is that the game's engine is optimized and looks VERY beautiful and it's not as demanding as Crysis or Grand Theft Auto IV. The second pro is that the game keeps the same feeling as the original and doesn't make any changes to the gameplay which is good because the gameplay in Serious Sam is fast, intense, challenging (unlike most FPS games these days) and really fun. The third pro in this game is that Croteam didn't use a horrible netcode for their multiplayer this time which was one of the biggest problems in Serious Sam 2 and they fixed that in this game (Thank Goodness). The fourth pro is that the game is only twenty dollars and the fifth and final pro is that there are a lot of Steam achievements which adds more to the game.
Now for the cons.
The first con is that you NEED Steam to get this game and there isn't anywhere else to get this game unless you have Steam. The second con is that this remake only changes the graphics to HD and for the third and final con is that this game DOESN'T have mod support. Which is a bummer because there was a lot of custom content in the original game.
Overall, this remake is great and for me i have only encountered a few bugs but there nothing game breaking. It's also twenty dollars which is great. However, if you were looking forward to make mods then i would skip this game but if you haven't played the original, then i would highly recommend this game.