ESP or Fishfinder?

Big DITTO in how just knowing the depth and structure below you can be a great aid in finding fish.   I can't say I have that much faith in the fish finding feature yet but perhaps more experience with it will help. 
I will also second pelagic paddler's comments on the Cuda 350 sMap
I enjoy the exercise aspect of kayak fishing.   So beyond keeping track of prime fishing spots,  I like being able to keep track of distance traveled and, at times, what my top and average speed is.    Kind of like a bike computer for the water 

I hadn't considered the extra benefit of tracking the exercise value of my day on the water...but I like it! Pushing my "freeboardless" arse to new speeds will be a welcome challenge too 
I have quite a few waypoints that I am confident I could access via yak that I can't WAIT to re-explore!

Don't forget having a gps also lets you know how fast you are trolling.

Hands down, ESP!
You don't need no stinkin lectronic falderal to catch feesh!


But it sure doesn't hurt though. +1 with everybody who said you use it to look at the bottom more than look at fish.

 Other than birds or fish working, the surface of all open water looks pretty much the same, wet and relatively flat. (Ok, you can see current seams, breaking rocks, etc. but you get the idea) Unless your in the Caribbean, your not going to see most reefs your passing over, let alone more subtle fish holding bottom changes.

 I like the Lowrance units. The M56 is the same as the Cuda 350. I also have a M68 color unit that I reeeally like. Color ain't necessary, but it sure is nice. The replacements for those units (Mark 4) look VERY sweet with upgradable charts AND a microSD card slot. That's hot because if you can save your trails with depths, you could build a 3d image of the bottom of your favorite fishin ground. The holy grail of fish mappin that I have been chasing for a while.

http://www.northwestkayakanglers.com/index.php/topic,1863.0.html

http://www.northwestkayakanglers.com/index.php/topic,4056.0.html


I also like the ff/gps combined as its on less thing to lose overboard.

Since a lot of the water I fish I have fished in a powerboat, I can only say that on a few occasions have I caught a fish due to the fish finder.  In the one case I can recall I drifted into shallow water while I was adjusting gear and my sonar lit up with baitfish.  Trolled there a few times and BAM a nice king. 

I will say that I believe my downrigger is going to IMMENSELY improve my fish catching this upcoming season and you just can't use a downrigger effectively without a sonar.  I guess you can but you risk snagging the ball on the bottom on line rated 50 pounds or more and that would NOT be any fun.  So in that sense I feel a fishfinder is essential. 

Not sure how it works in other places, but most places I fish, other boats and other people fishing are much better indicators of where to find fish.  I'd even list birds ahead of my sonar in fish finding ability.  So I guess my "sonar" is much more useful for me as a "depth finder" and not a "fish finder" if that makes any sense.