I did complete the Gears 1 single player, but never played the Multiplayer. The Single Player is worth picking it up for, especially as it is really cheap now. The same applies to Gears 2 in my opinion. The Single Player is one of the greatest I have ever played. Plus it can be played in Coop as well.
I really like the Gears 2 Online, the odd person hides behind a wall, but everyone else goes in all guns blazing. I find it really tactical, especially if you play Guardian.
Its all about out thinking the other team.
There has been a ridiculous amount of post release support from Epic Games, four major patches and four map packs. On top of that there is a new Single Player Chapter, and seven more maps, coming July 30.
Gears 2 should be reduced by now, I suggested getting both Gears 1 and Gears 2, you wouldn't regret it.