Pro It has an IDE used for loading all of the assets
It is very easy to manage all these resources you want to put in your game, the UI widgets for each assets (sprites, sounds, backgrounds, rooms, objects and shaders) are intuitive enough for when adding or even editing the properties of each your assets. The included editors are also good and easy to use (sprite/image editors, shader editor and room editor).
Pro Easy to learn
GameMaker: Studio is incredibly easy to learn. It requires almost no programming knowledge which means those without the technical experience, such as designers or artists, can get their idea made.
The internet is filled with tutorials for all skill levels for GameMaker: Studio. This makes learning it hassle free as there are great text and video guides out there.
The official tutorials are located here, and there is also a great tutorial by famous indie developer Derek Yu.
Pro Has a Marketplace
Yoyogames website has a marketplace which opens up an opportunity for people to sell or giveaway created assets and resources (sprites, scripts, sounds, extensions, full source codes, etc.) for use in GameMaker. This benefits people who needs quality assets for their games, and for creative people to provide these assets for extra income. The Marketplace has a rating system so it can eventually increase the quality and competitiveness of the assets submitted.
Pro Has a free version that still allows for creative game development
Game Maker Studio uses a tiered service beginning with a free options. With this option you have basic features that can still be used to make a decent game. If you decide this engine is for you, the first paid option is only about fifty U.S. dollars. This means you wont have to commit hundreds of dollars only to find out the engine is not what you need.
Pro Access to the GML scripting 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.
Con Destructive DRM
In late 2012/early 2013, YoYo Games released a version of their new Studio IDE for cross-platform development that would import games and destroy all of the image type resources for some legitimate purchasers of the software by superimposing a pirate symbol on top of the image. This was due to a fault in their digital rights management software implementation which they use as a method of combating pirated copies of the software. Though the false positives bug is reported to be fixed, the DRM is still in place and may affect placeholder graphics, etc. YoYoGames publicly stated they would remove the DRM at a later point in time, but that other less-invasive DRM techniques would remain.
Con The level editor is terrible
The worst level/room editor of all the game development packages. No marquee select, no layers, only one undo, glitchy, terrible manipulation of scale/rotation, can't manipulate draw order, no grouping, etc. It really is woeful and is surprising that anyone can use it at a professional level....truly is the weakest link in GSM!