OpenFiler uses IET for it's iSCSI implementation, which doesn't play very nice with ESXi - especially if you try to connect multiple hosts to it. You'll find the iSCSI target will spit out a bunch of errors and disappear if you push too much data at it. It's been an ongoing issue for years. Google "OpenFiler ESXi iSCSI Crash" and you'll see the forum posts dating back to 2009 or so.
OpenFiler works fine as a NFS store, though, so I'd recommend that.
I had an OpenFiler high-availability cluster set up at one time (a major operation to configure, by the way), and used iSCSI to talk to it from a pair of ESXi hosts, and it was just a total disaster. I spent a ton of time tracking down the problem and trying to fix it (some people hacked in the SCST iSCSI stack to fix it). I eventually just switched it OpenFiler to NFS, and the problems went away and all was well.
If you want iSCSI, use something else. StarWind Virtual SAN (it's free, even for 2 node HA, run on Windows, easy to use), QuadStor (also free, even for HA, runs on Linux, relatively easy to use), Open-e (not free, but based on SCST, so it works, and it'll do HA), Nexenta CE (free, but no HA options without paying for Enterprise Edition), etc.