The handwriting on the outside does make it look personal but, once you open it, it is obviously computer generated due to the consistency of the writing. If anything, I'd say the writing in inside undermined the message because nobody is going to handwrite a letter to solicit that business. You should test only doing the signature only and see if impacts your response rate.
We also got a couple of 'we want to buy your home' mailings. It does feel like it impacts our open rate but I can't tell if it is just due to novelty.
I think the main reason it 'works' as a business proposition is that the City is doing blanket increases and relying on protests to catch their mistakes. They do this (in part) because they are forbidden from using actual MLS data. It costs me 3-4 hours to do a protest so outsourcing it makes sense if your time is valuable. But when it came down to it, I wasn't willing to let them represent me -- something about it felt off -- and I wound up just accepting the increase.
The system doesn't feel broken -- just wasteful. How else are you going to do a property tax? I do wonder if this goes the way of medical billing (overcharge at first because you know you are going to lose some to protests) or it creates a watchdog effect that make the City limit the increases to an amount that is not likely to draw protests.