I am using msvc 2005 and vsaddin for a couple of years already. It is working and vsaddin has improved over time.
I have used qt creator scarcely. If I have to make a recommendation I am not sure. I am used to msvc, therefore, I am continuing with it.
If I did not know either, I would give qt creator a try. I has improved significantly since I first saw.
Note: you need a licensed version of msvc to work with vsaddin. The express versions do not support addins.
I love Qt Creator, as it cross-platform.
Visual Studio C++ express can be used too develop Qt application too.
Once you have a xxxx.pro file, you can work with QtCreator. If you want to use VC express, just run
qmake -tp vc
project files for vc will be generated.
I am using both =)
Qt Creator is very good for GUI building, creating your slots automatically, etc. I need to mention that so far I've only built desktop applications. Qt Designer is good but you'll have to do the coding by hand for your slots for example.
Now for the past 5 years I've been developing Windows applications with VS 2003, VS2008 and now VS 2010. I am used to have very good and detailed debugger.
While using Qt Creator I find the debugger too slow; also using VS, you can hover a variable and see it's content as well as content of vectors, etc. I didn't not find that with Creator and since for the living I'm developing imaging software with critical timing constraints, I believe Qt Creator is not the greatest candidate to develop all the modules with.
Now Qt Creator is cross platform and that certainly is a great plus. It's free and when moving to another platform for compilation it feels homey.
The VS add-in works nicely but when you have to jump back and forth between creator and VS, you'll have to add files manually. For example, if you add a form on Qt Creator, next time you'll open the project with VS2010 (in my case), you need to manually add the newly created form files to the project because the .pro file is updated but not the VS project. Little things like this and may or may not be disturbing in the long run.
Another reason why we'll still use VS is the number of excellent quality add-ons. At home I for example have Intel Parallel Studio, Nsight for CUDA, Visual Assist X, Visual SVN, etc. Most of them are commercial but there's a difference if you code for fun or the living. I doubt all those highly integrated tools are available for Qt Creator; I have not seriously checked though.
I have not yet developed handheld application with Qt Creator so I cannot comment for this type of development. I'm really eager to try out when I find time.
So my conclusion is that both are good depending on the context and that's why I use both.
I forgot. To open a project you created with Qt Creator, on VS go to menu, Qt, Open Qt project (.pro) and select the .pro file.
One thing I don't know how to change is that the Debug configuration has the code optimizations set for speed and that's why you won't be able to debug properly. So the first thing you have to do is disable the code optimization for all the debug configurations.