I used one for some time, and they are very good. You can turn your injection rate down significantly. Dont presume that the same bubble count you used with the diffuser is going to work with the up atomizer, it will probably have to be reduced. They are more efficient. However, they are probably best used on tanks greater or equal to 20g as on a smaller setup they tend to be overkill. Ive gone back to a difuser even though it means using a bit more gas. I found it very hard to get the setting right, and I found the bubbles a bit annoying.
I think the advantage to having the spraybar at the top is simply because the flow of co2 rich water stays longer in the water column. Flow goes parralel along the surface then down the front glass across the substrate then back up. If you put it lower down it can only go up.