One of the best ways is probably to watch this short video in which Hadi Hariri gives a demo of ReSharper's navigation and search, code analysis, refactorings, unit testing, code generation, and support for multiple languages and frameworks, all in about 10 minutes.
A lot more ReSharper videos is also available for you to view on JetBrains YouTube channel.
If you want to dive into ReSharper's diverse feature set, try ReSharper Online Help for an all-around description of language support, specific features, and how and why to use them.
ReSharper is way more powerful than even the latest releases of Visual Studio in terms of code analysis, set of refactorings and code transformations, depth of navigation support and in many more respects. For a brief overview of ReSharper value over recent versions of Visual Studio, see the following docs:
ReSharper helps with a great deal of frequent software development and maintenance tasks, such as finding unused code, complying with naming guidelines, detecting possible runtime exceptions, or adopting software design patterns. See ReSharper Cookbook for tutorials on undertaking these and other tasks using ReSharper.
There's a presentation called ReSharper Benefits (.pdf) that looks at ReSharper functionality from a ROI perspective and provides a basic way to estimate how a development team benefits from using ReSharper.
There's also ReSharper Data Sheet (.pdf) that fits the most relevant information on ReSharper into two print pages.
Every new major release is a result of a whole year's effort by the ReSharper team, and it's usually well worth the money spent. To learn what ReSharper updates add compared to previous versions, please see the following comparison tables:
ReSharper is a great tool for everyone. However, keyboard-centric developers get additional value out of it as it maps most of its actions to keyboard shortcuts. Use these cheat sheets to master your ReSharper keyboard scheme of choice:
Default ReSharper settings aren't perfect for everyone. For example, you might want to modify default naming rules or modify severity for certain code inspections. Refer to ReSharper Customization Guide for guidelines on these and other tweaks.
You can also use numerous ReSharper plugins that extend or modify ReSharper functionality, or even create your own plugins, and ReSharper Plugin Development Guide is going to help you with that.
