I dont have a flexi, but I do print white ink with my hm1. The basics of most machines are the same, so I will give my feedback. They are definately alot more work when adding white ink to the process. The time it takes to print is increase by alot, and the supplies of course add to that as well. But if you have a good market for dark shirts, and can get a price that makes it worth it to you, its nice
I myself started out from the get go with white ink, but most start as you are with dual cmyk. I would say to learn your machine really well first, so that you know how all the little things work, and what kind of maintenance is involved, then if you are comfortable, give it a try

You can always go back to dual cmyk if you dont like it, or if it is not profitable enough for you.