Without a doubt we can say that VS 2015 is the best release yet. Of course this is true for nearly all released. A new version comes with a new set of features, is adapted to the spirit of the time of its release and of course fixes some bugs. Nevertheless, as complexity grows a new set of bugs is certainly introduced as well. So there is always a caveat.
What makes Visual Studio 2015 such an important release is the integration of Roslyn and the .NET Execution Environment DNX. The latter is heavily used in ASP.NET 5 (sometimes called MVC 6, even though the distinction between ASP.NET webforms and MVC no longer exists as both can be used side-by-side if one wants to). Of course C# 6 is part of Roslyn and therefore available as well. Roslyn on the other side will definitely change the way we write code today. There will be a lot of analyzers and refactorings, which will be easier to integrate than ever before.
Cross-platform development is important for the guys at Microsoft. The VS team is aware of that and integrated popular cross-platform development frameworks and tools, such as Apache Cordova and Xamarin, into Visual Studio. Especially the former relies on the modern web. Luckily ASP.NET 5 also embraces the modern web with front-end build tools such as Gulp, package management over Bower or client-side MVC applications with AngularJS. The tooling understands these layers perfectly and helps us.
But also Visual Studio itself got better. It seems to be faster and contains useful information during debugging such as profiling times. Additionally we now have abilities like debugging, watching or editing lambda expressions or many other improved capabilities. The intellisense menu has been upgraded to support groups of names (e.g., such names with the same prefix).
With the community edition, which is free, Microsoft stretches their commitment to the open-source community. From my point of view VS seems to be nearly perfect now. If only it would be more modular and cross-platform. Oh wait, Visual Studio Code is also available! A bare editor, which is cross-platform and is already showing much better performance than GitHub's Atom. I guess the future is now!