I host 2 scan feeds and see quite a few listeners using it. (EmergencyRadio/1.1 CFNetwork/342.1 Darwin/9.4.1).
Another one I see used is called Police Scanner, but not sure which phone supports that one.
I have two feeds with ScanAmerica & I see that app listed all the time so a lot are using it.
Note: This app works on the iPhone & the iTouch. The iTouch is WiFi but uses the same apps as the iPhone.
My guess is there are more iTouch users vs iPhone as the iPhone will have to use the data plan & AT&T may have caps on data useage. The iTuch is WiFi so bandwith is less of an issue. But this is only a guess.
I use an app simply called "Radio" on my I-phone.
It works great for ScanAmerica. In fact the site is listed in their directory. very simple and easy. Volume is good.