I know that lens flare as a highly visible effect gets a bad rap, and with good reason. However, I have come to appreciate the fact that glare and flare are simple facts of life in all images. Even the human eye is subject to these effects, so no image that fails to incorporate them (even if they are very subtle) can appear truly realistic.
There are a number of papers describing how to implement this stuff, but from what I can see it would be sample-prohibitive to try and path-trace such things with Cycles.
Still, maybe there are such tools usable with Blender and I never noticed? Maybe someone has done a special add-on or implementation? Anyone know how to get really proper glare and lens flare other than faking it or using the glare node?