I am a professional video games developer, I have worked on many triple A console titles, mainly working in C++. But for the past year I have been using Unity professionally, at our company we investigated different tools and engines in which to prototype game concepts, and Unity stood out from all the rest.
I believe Unity is a fantastic tool for an introduction to game development, I'm perhaps very heavily biased but that's only because I love it so very much
1. Its fast and easy to see the changes you're making, be it to code or art. You save your texture, or model and tab back to unity, its straight away updated. With code you make your changes and press Play, no complicated build steps.
2. It's very little code (well technically, no code!) to get up and running, and very little code to do what you want. The libraries they have provided are really quite good.
3. The way they have an object hierarchy, with objects containing components (scripts you write) makes its easier to visualise what's going on. In most other game engines your code would have to create and manage all these objects and the main view into this world is from the code, but with unity you can see your code and your objects interacting.
4. There is a large support community (official unity forums, unity answers), and good documentation.
5. They can export their games to a web page, which will be playable from any browser with the Unity plugin installed (they are soon to add a Flash export option to make it playable through the Flash plugin).
6. It's free, you only have to pay for it for the more advanced features, external source control, real time shadows, post processing effects etc...
Theres been quite a few games written with Unity