This issue depends much upon a few factors:
Pre-existing tissue: The more natural tissue you have pre-operatively, the more of a chance you have of feeling "natural" post-operatively. However, if you have Cohesives or overfilled saline implants, regardless, your breasts will feel firmer than natural breasts.
Overfill amount: Too little volume will give you ripples, too MUCH volume will cause firmness and rippling. Discuss overfill amounts (percentages) with your surgeon pre-operatively.
Implant filler: Saline reportedly feels less natural than silicone gel or Hydrogel-filled implants.
Implant surface: Smooth-surfaced implants are thinner than textured-surfaced implants. Although very slight, patients having had both often report that they can feel a difference.
Implant placement: You may hear that unders look more natural than overs - when in fact, all cases are different. Overs actually move more naturally than unders but may have a pronounced upper pole fullness (especially in overfilled implants). Unders tend to "jump" and twitch when you use the pectorals during every day movements and working out. This may be a matter of opinion and preference rather than an effect which produces a blanket statement.
Thickness/thinness of skin: The thicker and springier your skin the less the implant edges will be felt. Thinner skin allows more a more palpable result. hence thin-skinned individuals often opt for under placement.