The effect can also depend a lot on the background you’re compositing over. For example, if you’re comping the reflections onto a solid color background it might look weird because we’re used to seeing reflective objects reflect a world and not just objects placed on them. So instead of rendering just a black reflective plane with objects on it, you might also want to add a sky object with a gray color or some kind of blurred image applied to it.
For example, here’s a render with no sky object:

And the same scene, but with a sky object on which I’ve placed a 50% gray material:

The object reflections aren’t brighter in the first comp, but they appear to be because the objects are the only things being reflected.