Personally I keep a .NET decompiler on my taskbar. I was about to say I like being able to find all references and usages of internal variables, but I see now the site does a pretty good job. I also like directly seeing how my code utilizes the library functions (not just the .NET libraries, sometimes nuget packages too). The decompiler also removes ambiguity of "what code is actually being ran". ILSpy has worked nicely for me, it has the extra perk of having nice integration with LINQPad (another must-have for .NET developers)
ILSpy + LINQpad is my go-to as well, and I actually still use ILSpy more often than source.dot.net. It's nice to be able to send a link to someone else though, and the site is well done, fast, and has a great domain. :)
