Benefits:
+ Free choice in display (Bigger, specular / not reflective)
+ Free choice in the input devices (mouse, keyboard, trackpad)
+ Easy Upgradeable (both RAM and hard drive)
+ Even quieter
+ Less power consumption (10 watts at idle)
+ Cheaper Price
Disadvantages:
-No Optical drive (an external BluRay drive I had already bought the iMac)
-No Camera and microphone (I have never used anyway)
-Less Computing power (In Normalanwendunen not noticeable)
-Langsame Hard drive (can be in contrast to the iMac replaced on your own)
-Schwache 3D performance the basic model (for a round Left4Dead2 now and then enough)
-Only 2GB RAM the basic model (8GB Samsung RAM cost 50 euros and are in 5 minutes without tools installed)
Now running my Mac mini with 8GB RAM and a -spiegelfreien- Samsung 24-inch monitor.
As input devices I use the Magic Mouse and Wireless Keyboard Apple.
They had convinced me the iMac.
Conclusion: The Mac mini can replace the iMac without any problems.
Only now I have saved a larger, non-glare screen, while at least 300 euros :-)