A lot of them are obvious. A lot of people look at iSCSI for cheaper connectivity. So when you consider that a traditional Fibre Channel SAN port for a director is oftentimes $1200 per port, and Fibre Channel switches can be $800 a port for customers, when you throw in $1500 for each HBA and then high-end storage arrays on the back end, the connectivity costs are significant for a customer who doesn't already have Fibre Channel infrastructure in place or isn't accustomed to spending that kind of money. The most obvious reason people are looking for iSCSI is to leverage cheaper connectivity.
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Another area that people are interested in iSCSI is for shops that don't have Fibre Channel infrastructure; they can leverage their existing knowledge in networking. They don't have to go learn about zones of VSANs, buffer to buffer credits and those things that are unique to Fibre Channel.
Another very important advantage is distance connectivity. It's not uncommon, for example, for developers to have machines under their desk, or in a remote location, instead of in the data center. They need the functionality of block storage and they want to have SCSI or local block storage and not NAS storage for their application developers. iSCSI knows no limits in terms of distance as long as there is network connectivity. You definitely can't say the same thing for Fibre Channel.
To play on the last point is that iSCSI provides SAN storage for all kinds of machines -- desktops to high-end. There are real advantages in those spaces.
Listen to the iSCSI vs. Fibre Channel podcast here.