I remember some tryout with PvP in DnD.
in 4ed it was a kill right in this first round, usually the healer.
My memories are almost one way fight. Even if both well prepared, one team make some slight errors, and bang, like in baseball playoff 14-3!
No dramatic turn over, no smart move, one mistake or wrong preparation and then steamrolling.
This is a very good point, even with good preparation and a stable game like 4e, it's a very swingy game, based not only on luck (a d20 is very swingy), but also really chaotic in the sense that it depends a lot on initial conditions (terrains, synergies, initiative, etc.).
Moreover, the action economy makes it obvious that as soon as a character goes down on one side, that side it more or less doomed extremely quickly. It's a phenomenon that we saw extremely well in our LARPs (we ran scores of very big LARPs with hundreds of players in teams of about 6 which were more or less equal in size and power, combat was not necessarily at the centre of it, but we also had teams of monsters roaming the woods with organisers in there, very well placed to see hundreds of fights). Some players complained that their adversaries seemed to win very easily, taking few wounds, while they were steamrollered. But from actually observing hundreds of fights, the action economy was absolutely critical, almost as soon as it's N+1 vs. N, it becomes 2 vs 1 and N-1 vs. N-1, the 2 vs.1 is almost instantly deadly and one team hits the ground before the other one can do much. The only mitigating factor in our games was magic, since a caster (or powerful knight/rogue with powers) could take 2 or maybe 3 adversaries and re-establish balance, but magic had limited uses during the game and skill in the end counted a bit, but it had to be when it was mostly on one side and little on the other.
So unless the DM intervenes at some level, I'm not sure that these fights can be really interesting, probably over very quickly if played cleverly.
And another thing which detracts me from them, the metagaming. How does the DM not metagame as he has all the stats of the PCs, and from the PC sides where they don't have it, they certainly can metagame from what they know about the adversaries. This is why I like my NPCs not to be built along PC lines, so that they can have the right abilities considering their role, but everyone is in the dark and in any case the DM is not playing against the players...