Is "Longest Road" an overrated strategy in Settlers of Catan?
Like many great games, Settlers of Catan rewards those players who are willing to be adaptable.

Doggedly centering your strategy, from the beginning, on obtaining wood and clay (to the exclusion of other goods) and building road after road with it, is likely to secure you a certain number of victory points.  That number is nowhere near enough to win you the game - you're going to have to do other things with your time and resources at some point to clinch first place.

Does the Longest Road bonus incentivise road building?  Of course it does, to some extent, just as the Largest Army bonus incentivises buying cards.  What it mostly does, though, is ensure that there is competition in various areas of the game.  If only one player cares about roads, the Longest Road bonus will fall to them without any effort on their part.  Likewise for Largest Army if only one player is bothering to buy cards.  However, gamer psychology is such that, given an obvious bonus to be obtained, players will not be able to bear letting it go easily, and at least some of them are likely to compete vigorously for it.

As such, I see the Longest Road bonus less as "a strategy" and more as a nice little, well, bonus... if you're brave enough to enter into a competition with other players over it.  Competition is necessary for a game to work, but it can, as you rightly point out, paint a general target on your forehead, or make enemies of specific other players.  If that's not the type of thing you like to get involved in, you might indeed want to adopt a more surreptitious strategy.

Certainly I would tend to prefer not aiming for the Longest Road early: let other people set themselves up as "the one to beat" by gaining an early lead.  Keeping an eye on the road situation, and perhaps setting yourself up to win with a late road-building spurt, is great though.  As I say though, adaptability is key: if everyone in your group fancies themselves as road-builders, it's a very risky strategy.  If no one else is doing it... you'd be a fool not to pile in there as quickly as possible, really!