It's quite like like a combination L2ARC + ZIL. However, the L2ARC isn't persistent across reboots (otherwise block writes would need to be journalled, adding complexity) so you don't get the speed benefits until the machine's uptime gets large. Not the greatest thing for laptops. Also, I believe the L2ARC is populated from in-memory pages, while it appears the Fusion Drive can be populated by a daemon process.