It's the only nipple we've used, per our LC's recommendations, so I can't compare it to anything else. It's still much, much faster than bf'ing, but my son easily goes back and forth between it and my breast, with little confusion and from what I can tell, he still prefers the breast.
It came with my medela harmony pump. I wouldn't have spent $20 for just a nipple though. She was okay with it for a couple weeks, then started refusing. By 2.5 months she refused every brand we tried so I don't think it was the calma that was the problem. We only used a bottle once a day. As far as ease of use she figured out how to use it right away. It seemed a bit long though, she would often gag until she got in the right sucking rhythm.
We've tried a variety of bottles and nipples and although our LO will eat from any bottle and nipple, we like the calma nipple because we can tell the difference in the way she sucks with that nipple as compared to the others. Her sucking motion is actually much more like how she sucks at the breast and since we have supply issues, the similar action works great for us since that means she doesn't have any difficulty switching back and forth.
Re: The new Calma nipple...
We tried the Calma but my LO prefers the Tommee Tippee brand.