11.6 - Personal bad experience with HF4. Had a recurring memory leak incident and an engineering hotfix did not help. However, many companies use this version in production environment, and have no complaints. I believe it comes down to complexity of configurations and the luck factor.
12.0 - No bad experiences, but I'd categorize this version as risky. A typical consulting firm would not recommend opting for a new major release which is only 6 weeks old. This is not an issue with F5 or this particular release, but I believe this risk factor applies to any "fresh from the bakery oven" software of any vendor. Perhaps 12.0 is worth a serious consideration a few months from now.