I'm an eternal beginner-level paddler in poor condition, who paddles 90% on flat water - and none of those things is ever going to change.
Having said that....
I do enjoy working on my stroke - having taken a couple of clinics.
In past seasons I had been using a Garmin 310 to keep track of my speed - but it was not all that useful because of the 3-5 second delay in showing the "Current" speed.
This year I have switched over to a SpeedPuck and cannot resist giving it a plug here.
SpeedPuck updates itself something like once every second and I am finding it extremely useful for reminding me about proper posture and stroke.... Lean back and speed immediately falls off, straighten the back and get off the backrest, and speed immediately picks up.... ditto variations in verticality of stroke and so-forth.
Bottom Line: SpeedPuck is a useful tool for the likes of Yours Truly.