I'm on the fence.  I tend to use `instance_eval` and `class_eval` unless I need to pass args to the block, and then I reach for `instance_exec` / `class_exec`.  That may just be because I have a history of working on gems that needed to maintain compatibility on 1.8.6 for a long time.  In the post-1.8.6 world, maybe it's best to prefer the `_exec` methods...