Saline implants placed above the pectoralis major muscle will look ripply, noticeable, obvious, and unnatural. With thick enough skin or enough breast tissue this may not be the result at first, but the experience has been that skin doesn't stay thick and breast tissue doesn't always stick around over time. Usually short term but nearly always long term, saline breast implants will look obvious.

Gel-filled breast implants can initially look natural above the muscle if they are sized and placed properly but will still very likely become noticeable as time goes by and the skin and tissues age. For a natural look, you don't want to see the border or bulge of an implant in the upper and medial parts of the breast. It's normal to have a bulge and an edge to the lower and lateral parts of the breast where the muscle doesn't cover anyway.

Experience has shown rather conclusively that all breast implants should be placed behind the pectoralis major muscle to provide padding, protection, and partial separation of the implant from the breast tissue. This provides a better result both short term and long term. 

The other advantages to sub-pectoral (below the muscle) placement of breast implants are possible reduction in capsule contracture rate and ease of obtaining mammograms with implants in place. The disadvantage or trade-off of below the muscle positioning is muscle pull across the lower part of the breast on actively flexing the pectoralis muscle. This can be minimized by release of the lower-lateral border of the muscle during the placement of the implant (dual-plane release).