Right now at the ZFS-using desktop PC I write this with:

640GB of total L2ARC, 66% utilized (34% available). 664MB of RAM is used to keep track of this.

The machine only has an uptime of a little over a week right now, but over that period L2ARC had a 93% hit rate.

L2ARC would fill more-completely but the machine is low on free RAM right now; Firefox is being a pig, and that means that some things get ejected from ARC before they can be sent to L2ARC.

It's generally quite responsive despite being a box that exclusively uses spinning rust as primary storage.

In terms of absolute reliability: RAIDZ2 is in-use, which allows two drives to fail without data loss. Adding the L2ARC did not change this: Either or both of L2ARC devices could die in an instant and the machine would keep moving along (at reduced performance).

(I'd planned to upgrade the hard drives to something bigger, which is a simple process with ZFS, and also add more RAM, but the market moved faster than I did and I'll be stuck with this for awhile.)