Recs.
Updated
SpecsUpdate
Pros
Pro Built-in support for pixel-perfect collisions
Enabling pixel-perfect collisions on 2D sprites is so easy many will likely turn it on when maybe they might be better off without it. Regardless, it simplifies small and/or retro console-like game work. This is a feature fairly unique in this class of game engines.
Pro Gives developers access to a more fine-grained controle over the logic through the Game Maker Language
Game Maker Language (GML) is the primary scripting language that is interpreted similarly to Java's Just-In-Time compilation used in GameMaker. It is used to further enhance and control the design of a game through more conventional programming, as opposed to the drag and drop system.
Pro Easy to find tutorials, examples, and help
GameMaker: Studio has a wealth of options and a long history of community-based tutorials and examples. The GameMaker Community features these for free use. The program also comes with a number of demos and tutorials. There is also a recently-opened Marketplace which offers free and sold assets for jumpstarting your games. And due to recent growth of YoYo Games, the Helpdesk system is greatly improved and will now be able to send you good, helpful replies to any issues you have quickly.
Cons
Con Difficult to learn
GameMaker: Studio isn't the most friendly for beginners.
It can be difficult figuring out how to get started, and the limited drag'n'drop feature means you'll likely have to use/learn some programming to create the game you want.
Con Now for 3D games
Game Maker studio 2 has gone 3D. It's no longer a 2D engine. Version 1.4 is buried deep on their website, it is obvious the makers really don't want folks to use the old version, but they also don't want to leave the old version users hanging.