Depending on what I'm germinating, rockwool tends to do pretty well. My only objection is that is holds a larger percentage of water (and lessor percentage of air) when wet. So every once in a while, I've seen it create damping off during unfavorable conditions. However, these properties make it great for cuttings.
Personally, I like Oasis foam cubes and wedges for germination of most crops. My last batch of seedlings was done in 1.25" rockwool cubes and every cube produced a healthy plant. I usually soak it in a 5.5 pH solution (I use R/O water and citric acid) for a couple of hours before I use it since rockwool's natural pH is around 8.
Rockwool is made from rock (usually basalt) heated up to 3,000 degrees F and spun like cotton candy. Most rockwool doesn't hold water and is used as insulation in structures (homes, etc.) in every country other than the USA (we love our fiberglass batts).
Just pop the rockwool cubes right into the soil and you will be alright. If your rockwool has a plastic "sleeve", take it off before planting. If you try and remove the cube, you might damage the roots and end up with an unhappy plant.
Good luck!
Also, haven't tried Rapid Rooters. Are they that much better than Oasis cubes and worth a try?