Last I used FreeNAS (back when I built my NAS over a year ago) it had the slowest performance of any NAS-based software I tried. This is why I did not decide to use FreeNAS and which is why I stuck with OpenFiler. It's quite possible they've addressed some of the performance issues in newer versions since I went down the path of testing the variety of them. I used the Windows Home Server, Server 2008 storage server, OpenFiler, and I forget what the last one was...I think it may just have been Server 2003 or 2008. All of which would barely ever break 50 MB/sec in my sustained large monolithic ISO transfer testing.
At the time, OpenFiler (64-bit) was the only one to make use of the 6GB of RAM in my NAS for file caching. From what Mercutio describes, it now sounds like FreeNAS is also offering the same functionality.
Despite what other opinions online suggested, I found it easier to manage and provision storage using the OpenFiler web-based interface when compared to FreeNAS, but many others have reported the opposite. I don't know why I found OpenFiler easier to use, but I have no way to suggest that it would be better or worse for you to try. It's not that FreeNAS was hard to use, I just thought that OpenFiler was more intuitive (relatively speaking).
As for using a software tool like FreeNAS as an iSCSI target for VMWare HA implementation, it should be possible. I do know of people using OpenFiler which is another reason why I chose to use it. I never did build my home ESX servers to begin testing with this, but it coming in the next couple months.
This video should give you a basic rundown of how to configure the iSCSI (assuming this is the path you want to go) when using FreeNAS with ESXi. I'm not 100% certain if FreeNAS will allow multiple host to discover, but you can just repeat the discovery step in the VI console on each ESXi machine for the same iSCSI LUN. With that, you now have a proper shared storage between two ESXi hosts and will need to proceed with the vmotion network configuration before you can do an actual vmotion or enable HA. If you get stuck with that setup, let me know and I'll help.
Now that I've seen the support, update path, and proper implementation of iSCSI of OpenFiler to be fairly non-existent, I'm going to go through the same testing once again after I acquire a decent Intel NIC for my NAS. There is a newer version of OpenFiler and also it may seem to be a newer version of FreeNAS. I also want to demo the newer Windows Home Server and find which one offers the best performance.