My first experience of the arena:
I created a unicorn, a giant mole, a dwarf (with full adamantine gear and grandmaster in most skills), an ogre, a giant tiger and a cyclops and let them duke it out. The giant mole died from blood loss after getting stabbed through the leg by the unicorn. However, it had managed to fatally wound the unicorn before this, so both critters died. This was after they'd been fighting for AGES, so both were covered in wounds.
The dwarf somehow had his weapon hand ripped off within seconds of engaging the other beasts, then his throat ripped out. He died from blood loss while the ogre was strangling him.
The ogre was being bashed on by both the giant tiger and cyclops while trying to knock off the dwarf, and after he had gotten about 20 different wounds (I'm not sure how damaging, don't understand the new wound system yet), finally dropped unconcious. The cyclops was in a similar state wound wise, as was the tiger.
However, the tiger was the only critter that remained concious in the end, despite having over 1 page of wounds. I quit while he was still knawing on the two other guys' unconcious bodies.
This took place over about 3 minutes. So, long story short, combat takes ages, but it's still fun
I had expected the dwarf (wielding full adamantine gear with an axe, all important skills maxed out) to massacre the others, but he was one of the first ones to die. Also, you have to press [ESC] to exit out of menus now, which is annoying. Now, onto fortress/adventure mode!
Edit: After reading through some of the other posts, my experience of combat seems to match up with everyone else's.