Doesn't always work. Half the time, it can't find the symbols and if you switch on CLR, even a 3GHz machine begs for mercy - the perf meter is up at 100% for ages.
VS6 modules don't work with VS7 and VS7 modules don't work with VS6. We've got VS7 C# linking into VS6 dlls. I don't know how they managed to get it to work but they did.
We have a system that uses both (management decision) and I normally have a message box popping up telling me which debugger to attach. If I attach the wrong one, that is the end of the debugging session and I have to start again.