Finished the Messiers and am working on the Herschels. I like hunting down galaxies and globulars and noticing subtle differences between them. I've found most of the Messier objects and some of the brighter NGCs. It will never compare to the subtlety of difficult planetary observing. Planetary and Lunar Observing Deep sky objects tend to be big, dim, and somewhat fuzzy.
Many astro geeks prefer the latter.
JPL consults me when they need to figure out what Voyager et al. are seeing. I'm a member of ALPO and regularly seek out fairly difficult observations. I've seen Encke's Division, Olympus Mons, and the Alpine Valley rille. I look at Mars whenever I get a chance, not just at opposition. I've looked at all the major planets in some detail and look at all of them when they are near opposition.