I sure do and has saved me many shots. You dont really "see" in b&w with the filter but it helps you decide when some elments might blend with each other. I like it a lot and never leave home without it.
Peak makes one, but is too dark, I recommend the ZoneVI one.
I use it and as Jorge said, I don't leave home without it. However, I like the Peak and not the Z6. They need to be dark to alert your eyes of the dynamic range of light. Harrison filters makes them also and I'd suggest that their blue one is the most accurate as through it you see what B&W film sees.
a company called SRB in the UK make the MonoVue viewing filter. It is a small round almost deep olive filter with a handle and an eyecup to block out stray light. It does take some getting used to and I find I can only use it outside in daylight (it's too dark for indoors) but it is very helpfull in studying the tonal values prior to exposure, and informing which, if any filters to use to aid contrast. I certainly use mine a lot.
I lost my viewing filter and being cheap found that if you take two pieces of processed but blank c41 film(from the film leader)or one piece and fold it in half it functions quite nicely as a viewing filter.I know it isn't exactly the same colour but it will give you some idea of the effect and help you decide if you need one.