Marcum units rock!!!! Especially in deep water, they ensure your's is the only flasher that's accurate. Get a Marcum within 1/2 mi of a Vex and the Vex user instantly becomes 2x greater risk of a stroke.
I've heard this same myth over and over again- I have a Vex FL-18 and my good buddy has a Marcum LX-7. Occasionally, I'll get some interference but nothing a push of the gain button couldn't handle. Love my FL-18, it may not have all the fancy gadgets but I added a digital depth display ($40-$50) and it does the job. I must say, my next flasher will probably be a marcum- the graph/flasher display is neat. Thanks Johnr- nature is neat too!