Gary,
I had a 172N with the H2AD engine. About 4 years and 300+ hours without any problems (my ownership started at 1400 hours engine time, and the plane had been sitting a couple of years).
I had all the lifters pulled at the first annual for inspection, along with the cam lobes, and found all to be OK. From my research and experience, if your inspection indicates no pitting on the lifters/cam and you fly the plane regularly from that point on the H2AD engine will be no worse than any other. At the third annual we pulled about half of the lifters and they all were perfect, so we skipped the rest.
Another shortcoming is this engine has a single drive gear for both magnetos.
The Cessna Pilots Association has a good article on the H2AD engine, I can get it if you would like a copy.
I ended up switching to fine wire plugs to solve a perpetual fouling problem (in spite of very agressive leaning).
Otherwise this plane was a lot of fun and I would not hesitate to jump at a great deal on another one.
