I only drink them if out and I just don't fancy a cup of tea or water, no harm in the odd one or two, but I would get a headache and feel bloated if I had more than that or on a regular basis.
I drink sugar free stuff with impunity. There are lots of theories about it, one being it encourages a sweet tooth and could possibly cause an insulin response. It’s up to you to decide what you believe.
Quick tip, don’t get the stuff from a pump. Not only is the stuff vile but it’s likely to be dispensed from tubes holding the sugar stuff. If you’re not sure, use your meter to test.