RAPID absolutely does not reduce lifespan or incur any extra write cycles at all, it's a caching system that uses DRAM to buffer.
Theoretically the highest performance benefit would be a high queue depth and small file size, where you will see bandwidths exceeding 4gb/s.
This doesn't happen in normal desktop load but there will still be a reasonable benefit when reading/writing smaller files.
Not too rare actually, for example if you're copying a bunch of ~5mb-~10mb picture files continuously you'd see a big benefit until the buffer is full.
The side effect is that it uses some of your DRAM and CPU. However, it only enables RAPID when you have idle DRAM and CPU so you won't see a performance hit if you're actually doing something.
The only possible negative is that it caches to the DRAM, then writes to the SSD so if you unplug your power after data has been written to the DRAM but hasn't transferred to the SSD yet, then you'd loose it. IMO not really noteworthy since you might loose it anyway if your power went out and you weren't using RAPID.