Hi Mike!

IMHO is the bigger part already done by you (moving from VC6 to VC2003).
The VC2005 compiler has also been improved to meet the C++ standard
conformance.
I don't see any big problems (if VS2005 SP1 was applied!).

For more info see:
Breaking Changes in the Visual C++ 2005 Compiler
http://msdn2.microsoft.com/en-us/library/ms177253.aspx

What's New in Visual C++
http://msdn2.microsoft.com/en-us/library/bw65k95a.aspx

If you don't use MFC/ATL, you also can use the free VC2005
Express-Edition to do some tests.

Generally, no.
From VC6 to the next version was a quantum leap in functionality and
standards compliance.
Especially porting template classes could be a real pain.

But from VC2003 to VC2005 was painless for most of my projects.
There are some extra warning and errors, but those should be very rare.
It should be a painless upgrade.

There is one grave exception: if your projects use Managed extention for C++
to use .NET functionality, you have more problems.
That has been replaced by C++/CLI, which is much better.
You can use the /clr:oldSyntax to cope with that, but it has caveats.

Other than that, I can really advise you to try it, since it should go well.
If your app only uses platform functionality and plain C++ code, you could
use the free C++ express edition with the platform SDK to try it and see
what happens.

Thanks for the great responses.  You've made me alittle more comfortable
about upgrading.  I've heard many great things about VS2005 but was afraid
to make the jump.