For what it's worth, I used Mudlick for about 2 years. It took about 3 months before I started to see a response, and worked very well.
After about 18 months the new customers dwindled, and the majority of the response to the mailers was for the low cost oil change we offered. These were repeat customers with multiple previous recommendations for basic maintenance, but would decline anything other than the oil change. Some would price shop and have work we recommended performed elsewhere, others wouldn't do it at all.
Overall I think it worked well, brought us a lot of new customers that didn't know we were here, but it reached a pint of diminished return. I stopped about 6 months too late, but I don't regret using them, and probably will again in a year or so. Now I plan to work a little harder on internet marketing and better targeting my existing customer base.