my understanding is that results from contesting your assessment are mixed. also depending on where you live, comps might not be so easy to find given that sales are down. if there are builders still selling identical homes at inflated prices that are boosted by incentives and rebates, that clearly doesnt help your case. not entirely comparable but there's still more than enough "housing prices havent gone down" nonsense out there for the county to make it hard on you. if you have a few neighbors in the same tract that sell, that would be perfect. and even though foreclosures and short sales ARE comps, housing bulls and the county i'm sure will claim that they are not.
i'm planning to suck it up for few yrs and then start thinking more seriously about this when i think the mkt has hit rock bottom. it might be interesting to estimate the costs of contesting (appraisals, legal, and your own time spent dealing with county bureaucracy) relative to your tax savings (net out reduced tax liability minus lost tax deduction). from there you can determine what sort of decline in easily provable home value you need to see before making it worth the effort of contesting.
i guess the housing bulls are right. real estate always goes up --<em> according to the tax man</em>!