Reasonably straightforward to swap over, and a good fit with Shimano cleats (although, not as precisely snug as original Shimano pedals). You'll need a 22mm socket that's slim enough to fit inside the pedals - a Tekton brand one worked for me (SHD22122).
I've been waiting for Vector 3 power on the SPD-SL platform. I had converted my vector 2 pedals to Shimano pedal bodies with the Garmin kit and some Shimano pedals. Once I upgraded to Vector 3 it was back to the KEO platform which I don't like. This is a pretty easy install and only took about 30 minutes to complete. The Rally pedal bodies have metal threads vs the plastic threads on the Vector 3 pedals. I plan on buying a set of the xc pedals for my cross bike. It's easy enough to change them as needed for the cross season.
I had initially purchased the Garmin Rally XC200 pedals and were very pleased with them. So I purchased the conversion kit. The process of converting is easy and straightforward. I had been using Powertap P1 pedals and like the Garmin's a lot more.