CTJ's anecdote is probabaly the experience of 50% of people that protest their properties in person. I did my own in person for a couple years because i felt like I got better adjustments than going through virtual settlement. Not sure if that is still the case or not.
Anyways - I put together this big binder with comps, adjustments to the condition ratings of my home, replacement/repair costs, and adjustments to other comps for things like pools etc. Justified something like $150/sf down from $190 or so. They basically disputed/ignored all of my work, and then granted me what I wanted anyways because they "liked the neat binder I put the materials in." Then it hit me that probably half the people that protest themselves just show up to yell at them, and get nothing.
In my latest home I will probably do isettle, but using protax or bettencourt or whatever is better than nothing. I'm in a situation now where I'll be getting the 10% capped increase to appraised value every year until it catches up to market value so protesting wont save me any tax dollars now, but will help in the future.