In a word, don't do it.
It's an absolute mess. The entire C standard library has been declared as deprecated which adds a million warnings to your builds. Linking with vc80 runtimes is a major pain in the ass and converting your project from 2003 or MSVC 6 to 05 is equally as painful.
So far I've attempted to port my DLLs and my engine code to no avail. MFC also does not work. Always get application configuration error even doing what people have told me to in order to get it to work.
C libraries are no longer linked statically but dynamically and one website even suggested copying the libraries into the same folder as your project. This is a huge mess and I hope they sort it out soon.
Lots of people, according to what I've seen on the net, are having a lot of trouble porting from 6.0 and 2003 to 2005. I was going to port my recent projects over but I'm going to reinstall MSVC 6 because I have no time to sort out stupid manifest issues when I know the code works.
My advice - don't port anything of importance over to 2005. Finish it with what you have and then use 2005 for brand new projects.
My buddy at at&t has also had problems with porting VB 6 and 2003 over to 2005, and just even porting 6 to 2003.
I would not recommend it.
Will some company please step up and save C/C++ before MS destroys it or turns it into one of their own crap-ridden bloated code bases?