I have been using gennies for about a year and a half, beginning in the fall of 2011. Once you figure out how to build them, they work really well. I have been able to get all of them (DiD clones, AGA-Ts, real DiDs, GTUS, etc.) working perfectly. Great flavor, great TH, etc.
However, they do have a high fiddle-factor, even after you've figured out how to get them set up. It doesn't take much to throw one off, so that you have to take it apart and fix it. There is pretty much daily fiddling, adjusting, etc. I didn't really consciously notice this until that "mesh will kill you" thread that came out a couple of months ago. At that point, I started looking at other options, at which point I discovered cotton. Since then, I've been using cotton in drippers (mostly) and now have a Spheroid and an A7-oid (it also works well in tanks like the AGA, but with somewhat less flavor).
The cotton (or silica/hemp/etc.) provides many advantages. Setup is super-easy compared to a genny. There is no fiddling whatsoever. Performance is on par with gennies (flavor, TH, vapor, etc.) and even better in some cases. The drawback is that a cotton wick only lasts a couple of days. However, replacing it is very fast an easy - maybe 3-4 minutes, and that includes buring the coil clean. Pull out the old wick, burn the coil (been using same coil for weeks), insert new wick. Easy as pie, and you have basically a brand new setup. I spend a fraction of the time setting up and using cotton wicks compared to gennies. Also, the Spheroid and A7-oid never leak. Gennies can be set up with limited leaking, but it's hard to get them totally leak-free and - again - you have to constantly fiddle with them to maintain that status.
Now, does all this mean that I hate gennies or will will never use them again? Obviously not, but for ease of use and convenience, I'm definitely gravitating toward the cotton (silica/hemp/etc.) devices.