It depends on your purpose. If you program for the Web, avoid indexOf, it isn't supported by Internet Explorer 6 (lot of them still used!), or do conditional use:
if (yourArray.indexOf !== undefined) result = yourArray.indexOf(target);
else result = customSlowerSearch(yourArray, target);
indexOf is probably coded in native code, so it is faster than anything you can do in JavaScript (except binary search/dichotomy if the array is appropriate).
Note: it is a question of taste, but I would do a return false; at the end of your routine, to return a true Boolean...
