I've been running for about a year with no problems but when I did the installation I had to re-create the ISO to include the Ethernet driver to work. Will migrating to 5.5 break the Ethernet connection again or keep it after the upgrade is done? I'm only using this so I can run a few operating systems and really didn't need a full blown server, that's why I'm using the Dell Optiplex 790. The 790 comes with:
Intel 82579LM Ethernet capable of
10/100/1000 Mb/s communication
It looks like ESXi 5.5 still doesn't have the network driver included which means your network will go down, again.
I did a quick search and there are several people who have issues with ESXi 5.x and Dell Optimex 790 NICs.
According to VMware KB: Methods for upgrading to ESXi 5.5 upgrading (meaning upgrade "in place" using CD/DVD/USB or upgrade via VUM) from ESXi 5.1 to ESXi 5.5 does maintain custom VIBs, so you may be in luck and the driver may be "migrated" during the upgrade such that you don't have to install it again.
Just in case the driver isn't kept or you decide to reinstall rather than upgrade in place, there are a few options you can look into to determine how to proceed with the ESXi 5.5 reinstall.
Good luck! Please let us know how it goes.