You will never want to switch back to your EVA after you use a 3PAR.
I have a P7400 in the lab at work and it's great. iSCSI and Fibre Connections are a nice addition over an EVA. The chuklets will make your environment better.
It certainly feels like a huge improvement, but I don't have the statistics to back that up yet. I know that I'm using lefthand, EVA and 3PAR in the lab and my higher performance stuff goes on that 3PAR.
HP is shipping them now, but there is pretty high demand from what I understand. I wouldn't be surprised if you have to wait a few weeks before it could be shipped.