I've used both 6 and .NET. Here's a summary (and I got it for free at one of the Academic events!)
The IDE is largely improved. Debugging (step to cursor) is much easier now, once you find it. Right click on the line and there you go.
Resting the cursor over a variable or function will show it's type.
The help is integrated better into the system.
You can use different languages together - this means that projects are now called solutions, which can consist of several projects.
The "autocomplete" sometimes did not work for pointers to objects for me in VS6. This works flawlessly now in VS.NET
Cons: It's TOUGH to get used to it. I am still spending a lot of time looking for certain functions.
Summary: If you just program in one language, VC++ or VisualBasic, and you don't want to get used to a new IDE, that is VERY different, then stay with VS6. Otherwise .NET is the way to go. I don't regret updating.