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 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.
Pro Has a free version that still allows for creative game development
Game Maker Studio uses a tiered service beginning with its free option. This standard edition of GM Studio has all of the features of the professional edition, minus the additional export modules for cross platform releases. The free standard edition also comes with a non-removable "Made with Game-Maker" splash screen. All other features are unhindered by the free version.
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.
Con No built-in refactoring tools
There are no built-in refactoring tools - for example, you can rename a resource, but GM:S will not automatically change the mentions of it across the code to the new name. At the same time, however, all file formats are text-based, meaning that the effect can be achieved by simply doing "find & replace in files", which is a feature offered by pretty much every external code editor nowadays.
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.