My $0.02.

I think the name "fish finder" is a misnomer.  What it actually is is a "depth finder."  Yes, most do mark fish but I've never stopped because it marked fish.  I have stopped to fish when I find some irregular structure or particular depths - particularly mid lake.  It's a shame that so many people that want to catch big or many fish will paddle over very fish-able water.  Knowing what depths fish are typically at at any given time of the year is the key.  I recommend one if fishing is high on the priority list.  We've paddled across some big water marking 100+ feet of water only to watch the bottom of the lake come up to 12-15' in a few paddle strokes.  That's when I drop my jig and the fun begins.  It also helps you stay in that targeted depth zone along the shore so your time will be more productive.

+1 to what PJ said.  I pay little attention to whether or not I "see" fish on the screen. 

I also find it saves me lures and hassles by avoiding getting hung up as much.  I bet I've saved close to as much $$ on not losing so many lures as I've spent on a depthfinder.