My development tools are currently Visual Studio 2008, Resharper 4.5, Gallio and Testdriven.NET. But for as long as I have used these tools, my Visual Studio debugger has only worked partly.
The problem
When I inserted some breakpoints in my code, the first one was almost all the time hit. However, when I tried to step through the code, it worked fine for a bout 3-6 steps. But then Visual Studio debugger decided that was enough, and just completed the code.
The good thing was, it forced me to write many unit test cases, but sometimes I really think it is nice to visually see the state of your objects, in the code.
I have spoken to colleagues, and tested with different combinations of Gallio, Testdriven.NET and Re-sharper enabled, as I thought they cased the problem. But the debugger just kept bugging me. I had actually given up finding a solution to this problem, thinking that Visual Studio 2010 would fix it.
The solution
So finally, in a complete other context, I stumbled upon this page:
http://code.msdn.microsoft.com/KB957912
And thought…. that title sounds interesting… I quickly moved to the KB article, and started to read. I thought, that sounds exactly as my problem.
With nothing to loose I installed the update, and started to test the debugger… until now it seems to work 
- please note, I do not use the debugger that often as I write very reliable code
We struggled with the same issue on my dev project with the same resolution (after pulling out a fair amount of hair). You can rule out Gallio and TestDriven.NET, we don’t use those and still had the issue.
I am glad to hear you solved it as well. I had been coding for 6 months without debugging… it is very nice to have that functionality back