I've used a ROK at a friend's house. It's relatively inexpensive (he paid in the neighborhood of $200 for his) and fairly easy to use; but if you're used to an excellent espresso, you'll notice the difference in taste/quality of finished product.
Drawbacks:
1. TAMPING: Tamping the grounds properly in the portafilter is crucial to a good pull. The grounds should be leveled, then about 30 lbs. of pressure applied with a good quality tamper. With pressure still applied, the tamper is rotated to polish the top of the grounds. This ensures the water will go through the grounds evenly, extracting full flavor. Under the pressure of a pull, the water will naturally look for weak spots in the tamp, taking the easiest route through the grounds, compromising flavor and body. The ROK's flimsy plastic scoop/tamper is not up to the job.
2. TEMPERATURE/PRESSURE: A regular expresso machine heats the group head and portafilter. You can do this manually with the ROK, but it's a messy extra step. It's also not easy to consistently control the pressure in which the water goes through the grounds. I found the shotl thin/watery, the flavor only so-so, and the crema lacking body.
3. STEAMED MILK/FROTHER: The ROK comes with a manual milk frother, but it doesn't work well at all, and you have to heat the milk separately. Don't expect the rich, thick head of frothed milk you get from a regular espresso maker's powerful steamer.
It's a cute little machine, but I wouldn't want one.
[this post was last edited: 7/31/2014-23:01]