I like cubes (especially light or medium toast Hungarian oak) because that's what I'm accustomed to, they are easy to bake or steam-sanitize (a must do step for me), and ultimately because after 20 years of using them, I know what to expect from them in terms of quantity and exposure time. Whatever you use, it does take some experimentation to find that "sweet spot".
American oak is fine but you do need to be especially careful with it...it's so easy to overdo it.
The key with any oak though is to take it easy. Too much is really cloying (though of course, it boils down to personal taste in the end...you really have to find what you like). But I've tasted some very good homebrews that would be great ones were it not for overdone wood character.
The secondary is a good place to add the wood, though if you have keg equipment, that is really the best place to do it. You really do need to monitor the infusion of the oak flavors though. Too much really can be quite unpleasant.