I have the hummingbird 899 with both down and side scan. The problem with using it for trolling is that there is a lot of water between you and the bottom, so you can't see a fish on it unless you're in shallow water or really zoom it in. I have the unit, but only use the old standard sonar function when trolling. If I'm fishing 30 ft anchored for perch, then I'll turn on a split screen so I can see sonar and scan images. If you're looking to just use for deep water trolling, stock with just a good sonar unit. If you have the cash and want to fish shallow and see high detail (like a picture of what's below the boat) then go ahead and spend the extra dough to get the down and or side scans. I'd recommend getting out with a couple guys that have these units so you can check em out before you fork out close to a grand, and don't just get wooed by the screen shots or simulated images on the web or in the store - you wanna see how they actually function in real life and fit to your type of fishing.