In no particular order:
-Travelling upwind with total ease. (something I find a challenge on slalom gear) Being able to explore my lake instead of just reaching over the same patch of water for 2 hrs.
-The fact that you are mostly flying all the time. Many (quite good) slalom gear sessions are blighted by periods of non-planing if the wind drops under 15kts. Once you are in possession of a foil these periods are hard to take.
- Wind range - Being challenged in 10kts of breeze! I have had some of my most satisfying foil sessions in these conditions. Being on the edge and doing 25kts board speed or so in these conditions is a massive buzz. (Ok, big sail, race foil, 8.4 but light and much easier on a foil).
- Efficiency? - For me, slapper sailing, as thrilling as it is at times, can't match the clean efficiency of a foil. It has changed things for me. My slappers just feel draggy and difficult now. (in saying that, I'm not that good and I am probably just not doing it right)

- The buzz! - It's definitely got the wider windsurfing community buzzing again(at least in my area!) People are coming out of the woodwork to give it a go. Many a "lapsed" sailor has approached me on the beach asking about them. Can't be a bad thing. It may be a fad but time will tell.