99212 ("level 2") is the lowest-level visit a physician would bill for an established outpatient visit, and is essentially a single, simple problem requiring minimal ROS, an extremely limited exam, and no prescription drug management (e.g., advice or OTC treatment only). I rarely code those.
My average visit is a 99214 ("level four"). The only difference between a 99214 and 99215 ("level five") is risk. If a patient is complex enough, or their problems are serious enough, you can justify a 99215. You just have to make sure your documentation meets the requirements for whatever level visit that you code.
The biggest decision point for most primary care visits is whether to code a 99213 or 99214. This article may help with that:
http://www.aafp.org/fpm/20050900/52codi.html