I know this is a lighthearted question, but it is also an important one. I find that there really is a line to consider when indulging a passion. The dark side of a deep interest in anything is that if there is not enough discipline to it, it can consume our attention in ways that take away from our life rather than enriching it.
This hobby has incredible things to offer. We also have endless ways of escaping our lives, feelings, and relationships, and metal detecting can certainly become one of them. For me, the key is not choosing detecting over time with my wife, kids, and friends in any way that really compromises things, and being diligent about setting internal limits with myself for how much time I spend thinking about it. I go out 4 hours a week, unless I have some vacation time. When I'm aware that I'm getting unbalanced (like when, instead of seeing all the beautiful forest and farmland around me, all I can think of is what is underneath all that ground) I take some time off.
I can only speak for myself, but I work actively to keep that balance with this hobby; I could so very easily become some obsessed, bearded critter lurking in the forest for that next score!