It is a very good question to ask. There was a multitude of studies done to find out if there were any saftey issues with silicone implants. None of the studies were able to show any significant association of silicone implants with any particular disease or problem. Thus, silicone implants are considered to be very safe.
One of the complications of breast augmentation, and the one your question alludes to, is something called capsular contracture. Once an implant is inserted into the breast, the body forms a scar around which we call a "capsule." Usually, this stays soft and does not cause any problems. However, in a small percentage of women for unknown reasons, the scar starts to become hard and tight. When this happens, it is called capsular contracture. When it happens, it can cause the breast to feel hard and look distorted. This can happen whether you have saline or silicone implants.
It can be taken care of if it happens. If capsular contracture does occur, the solution is usually to re-operate. The incision used in the first operation can usually be re-used. Then, all of the scar and the implant is removed. I prefer to replace the implant with a brand new one. Once the hard scar is removed, the breast usually remains soft. Rarely, in a small percentage of women, it can happen again.
In summary: Silicone implants are safe. All surgery has its complications. Capsular contracture is one possible complication of breast augmentation. It can happen with both saline and silicone implants. The vast majority of women who get breast augmentation do not get capsular contracture and are very happy and satisfied with their surgery.