Tom started it with a joyous description of the Fein. Don't have a Fein but have a Dremel Multi-Max due to budgetary limitations. Love the tool.

For cut-outs in fiberglass I had often used carbide router bits but they did not enjoy the work. My son is a process engineer for high-tech machine shops was here one time and suggested using carbide end mills. The up-cut flutes bring the material up out of the groove - use a dust collector - and they outlast the wood duty bits by several times. Here is a link:

http://www.the-carbide-end-mill-store.com/m5/120-3250--1-4-square-end-mill-standard-length-htc-120-3250-3fl-gp-30-uncoated.html

The only problem I have had is the shiny slick shanks don't grip too well in a router chuck so scuffing them just a little bit on a diamond sharpening stone gives them some grip. I think this is probably similar to a RotoZip tool which I have not used. BTW I have tried them on wood but the up-cut leaves a fuzzy edge on the upper edge of the cut so I use my (now) non-abused router bits for that duty.