Useful perspectives -- thanks, all.
Since I posted this question, I've been chatting with some local coworkers/friends and have found many people who have successfully appealed their assessments... but ONLY with a legal firm. I was surprised, so I reached out to the Realtor we'd most recently worked with (since he owns several properties in the city and has always struck me as pretty knowledgeable) -- his take was that the process is stacked in Cook County (Chicago + surrounds) such that going with a legal firm is necessary.
I'm interested in the stats in the Trib article you posted, YttriumNitrate, because the success rates for DIY vs w/ a firm appear to be so similar... which IS what I naturally expected, but IS NOT what I've now heard anecdotally. (Though I'm totally willing to accept that my sample may be very biased.)
We're talking about a big potential hike -- nearly doubling my assessed property value, for a tax impact of thousands of dollars a year. Since this is SUCH a big jump, and since it's a triennial assessment that will stick with me for a while, I'm becoming inclined to go with a legal firm this time, and try to DIY it in the future when the stakes are a little lower. (Plenty of firms vying for my business, with $0 at risk if no reduction is attained; fees for success range from 30-50% of one year's tax savings. So not small, but the assessment is set for 3 years so the savings or lack of savings in years 2 and 3 continue...)
OY.