Recs.
Updated
SpecsUpdate
Pros
Pro Easy to learn
It is one of the biggest and best tools available, and there are a great deal of tutorials available to help learn how to make great games. Even without knowing how to program there are assets in the asset store that can help with that by allowing linking scripts inside the game engine.
Pro Art Asset pipeline supports a lot of tools
Assets can be imported from many different formats including .FBX, .OBJ, .Blend, .Max.
Pro Great editor
The editor GUI is very powerful and intuitive. It allows pausing gameplay and manipulating the scene at any time as well as progress gameplay frame by frame. It also has powerful asset management and attribute inspection.
This allows it to be more powerful than other, simpler drag-and-drop engines such as Game Maker Studio, although it can take a bit more experience to learn the workflow.
Pro Used by indie and big name studios
Games have been made in Unity at all levels of the Video Game industry. If you want to make games, this is a great engine to learn. Learning Unity will teach you the basics for any engine, and if you want to get a job at a big studio there is a chance that you will be working in this engine there as well.
Pro Allows a choice of scripting languages to use
Unity provides a selection of programming languages depending on preferences or knowledge. C# (CSharp) is arguably the most powerful, although the majority of tutorial languages are for Javascript. It is probably best to stay away from Boo (which is a flavor of Python).
Pro Well structured
Overall, a coherent engine with a rational approach. People who complain a lot about being forced to hack around it usually do not read the docs, like the one that describe orders of execution, or specific functions hooks and such. Some like to say it lacks raw power where people who are used to standard optimizations have no problem. For example It is not uncommon to encounter users who complain about low FPS but forgot to activate occlusion, flag static elements, activate animations culling, and so on. As for complaints about C#, people who are transitioning from C++ were already bad at C++ before being bad at C#. They often come from the PC world where the sheer power of today's machines is very forgiving compared to the platforms we had to develop for in the 80s~90s. One of their errors is for example to never read this doc.
Pro Lots of assets can be found in the Asset Store
For those developers who can't afford an artist, or aren't skilled enough to create their own art, Unity features an Asset Store full of a wide variety of free and paid assets that can be easily added to a game. The Asset Store has more than just music and art. It also has code and modules that can be added to games including unique lighting or GUI systems. It also has powerful asset management and attribute inspection.
Pro Lots of resources to learn from
Unity3D provides an exhaustive documentation where everything is given a full description supplied by a number of examples as well as video and text tutorials and live training sessions to understand the ins and outs of the engine. In addition there's an ever-growing community that can offer advice to help resolve any situations that may arise.
Along with the official Unity resources, there are many high quality (and often free) third party tutorials available.
Cons
Con Encourages bad coding practices
A lot of Unity code feels like a hacked blur of arguable coding practices. I really like C# and .Net, but the way it is used in Unity makes me feel uneasy. A lot of the API is done in "C Style" (public static methods, available at all times), I am encouraged to use public fields for everything, a lot of questionable implicit casting... the list goes on. I'm a little "scared" that a lot of people will actually learn C# by using Unity.
Con Very self-centered engine
Unity3D uses very unique approach for doing things, most of knowledge acquired while using it, would completely non transferable to other engines. Advanced Unity3D programming is really dealing with Unity3D bugs, and finding loopholes around engine issues, nothing to do with graphics, etc - skills which would be valuable with other engines.