It makes your SSD benchmarks look really good.
In reality, as I understand it anyway, it uses RAM caching as a way to speed up read/writes. There are a number of posts here about, give search a whirl if you want to read more about it.
Personally, I don't use it, outside of benchmarks I saw very little benefit, and there may be some risk to your data if your system crashes while writing (although you could really say that about your entire system, anyway, RAPID or not.)