Resharper is much better for C# code (and supposedly VB.Net, but I haven’t tried that).
Unfortunately there is no support for C/C++, so if you need that, you might want to keep Visual Assist around.
They don’t coexist very well, unfortunately, so you may need to unload one, then load the other, when switching between C/C++ and C#.
To see the magic of Resharper, I would recommend watching the “Resharper Jedi” video.
C/C++ = Visual Assist X.
For me, C# = ReSharper + Visual Assist X.
Needless to say, ReSharper is much more powerful for C# development, than VA. But there are some features, like ability to show just methods in suggestions list, or highlight nearest scope that are in VA, but no equivalent in R#. I use both. Looks like they live pretty well together: i use default settings for VA, and i had to select ReSharper–>Options–>IntelliSense–>General–>Visual Studio to enable VA version of IntelliSense instead of ReSharper’s one.
I also customized identifiers colors in ReSharper, now they look like VA default colors, but show additional information (like Mutable Local Variables are bold).
I know you only asked for a comparison of Resharper vs. Visual Assist but if you are doing .NET development you may also want to consider “Refactor! Pro”.
I remember using VA years ago when doing Visual C++ development (and earlier than that the infamous CodeWiz) but with .NET development I get the impression that the majority of developers seem to use either ReSharper or Refactor!.
Refactor! also integrates with a code-generation tool called “CodeRush” and I’ve seen them both used very effectively together with Testdriven.Net (check out the Summer of NHibernate screencasts).
Personally I use Resharper and I am very pleased with how much it has increased my productivity but I’m sure you’ll get equal benefit with Refactor!.