For my taste, a Roto-Zip style tool seems a little too specialized — there's not a lot things it does well except cut drywall and paneling.
I have a Craftsman electric die grinder that I use with:
1. Roto-Zip bits to make fast holes in walls (uses the same 1/8" collet size),
2. Dremel attachments for small grinding, polishing, etc. (comes with 1/16" (?) collet adapter)
3. A 3" cutoff wheel for cutting sheet metal, body panels, etc. (handles MUCH beefier jobs than a Dremel would)
4. Various rotary files and grinding stones for porting manifolds, cylinder heads, etc.
The die grinder is certainly heavier and bulkier than a Dremel tool, but it's built to last (and didn't cost much more, either). Biggest disadvantage is probably the lack of variable speed (although some other models have this feature).