I've been an ESRI user for about 10 years and have been using Autodesk for a couple years now. At work we use both and have users that are more comfortable in one or the other system. Here are few of my observations:
- Map 3D 2007 is nice in that it connects directly to ArcSDE data so AutoCAD users and ArcGIS users can edit the same data. This allows for more precise (and intuitive) edits using the CAD tools rather than ESRI's "sketch"-based edit tools, which are ridiculous. I'm only starting to use the SDE feature in Map 3D, but it seems really promising. Map 3D also supports versioning of the SDE data.
- If you work with imagery, you will find the image transformation tools in Map 3D are pretty sparse. Unless there are better tools buried away somewhere in Map 3D, a 2-point transformation or rubbersheeting is all you have. I use ArcGIS for image work.
- Map 3D offers some other GIS features like topology creation and various overlay tools, but generally for this stuff I stick with ArcGIS. I'm going to try out Autodesk's take on topology though, because I've found geodatabase (ArcGIS) topology to be mushy and undependable compared with its implementation in coverages (Arc Workstation).
Hope you find this helpful.