It's one of those games that you could buy, play once, and leave in your Steam list for years before getting interested in it, or you could jump in right away, depending on how you feel. It really isn't easy to start off, but with just a few skills you'll usually manage to gain a handhold, and from there onwards it's just a case of 'try X tool with Y object and see what happens'.

Most of the appeal doesn't come from playing alone but going online and finding a server or a community to stick with. When you can log on and see what people are making every day, when you can pick up cool ideas, and work on your own project... well, the feel of a good Build server is almost Minecraftian. When you're not in the mood for Build, there's an extensive number of mods for Gmod, most of them more sophisticated remakes of CSS mods - roleplay, surf, deathrun, jailbreak, Trouble in Terrorist Town, survival, onslaught, and spacebuild are the ones that immediately come to mind.

Is it worth it? Hard to say. The game is only ten dollars and depending on how you receive it, you could get hundreds of hours out of it or get bored after thirty minutes. I know I got the former out of it, and I figure that was worth the price and effort.