I have the dual beam (9 and 19 degrees) ducer on my vexilar FL -8. This really is a neccessity. I use the 9 in any water deeper than 25 feet because you can use less gain to see your jig. Given that I fish mostly for lake trout, I use the 9 most often since I fish 30 - 50 foot for lakers.
The 19 has it's place too. I use it in shallower weedy lakes when chasing pike and panfish, or when trout or walleye go shallow. The 9 has too much power to be used in lakes with a lot of weeds in shallow water, you will get too much signal.