Silicone Implants are Safe
It is smart to do some research before you undergo a procedure.
There have been numerous long term studies of silicone breast implants. None of the studies have been able to show that silicone implants cause any kind of illness. The implants themselves are safe.
If someone develops a problem, it is not because of the implant itself, but because of something related to the surgery. One problem is called "capsular contracture." This happens when the scar around the implant becomes excessively tight and begins to distort the appearance of the breast. This happens to 5-15% of women and nobody understands exactly why it happens. Most of the time, it can be taken care of by removing the tightened scar.
Other problems like infection, bleeding and wound healing issues do happen also, but these are rare. Plus, these are problems that happen with any kind of surgery, not only breast augmentation.
The most common issue is probably dissatisfaction with the result. Some women may want a bigger size or a different shape. This can be avoided by a careful and thorough consultation with the surgeon. You should be able to describe exactly the kind of result you want. Also, the surgeon should explain exactly what he/she thinks can be accomplished.
It is extremely rare to have some kind of debilitating complication from having silicone implants.
In general, if you find a nice doctor, with good before and after pictures, who listens to you and is Board Certified, then you will probably have a very positive experience and result.
Silicone breast implants safety
As Dr. Edwards stated in his reply above, the safety of silicone breast implants and their lack of any proven connection to diseases and health worries such as auto-immune disorders has been clearly established.
If you go to http://www.breastimplantsafety.org, you will find the most up to date and reliable information regarding silicone breast implants.
The data is in from our FDA as of 17 Nov 2006 that they feel, after 14 years of clinical investigation, that silicone implants are safe. Having said this, they are not for everybody. Although I use silicone implants in 80+% of my breast patients, there are some who for what ever reason, don't feel comfortable with silicone. I look at my job as a plastic surgeon and patient educator to make sure you know what your options are, the risks/benefits/alternatives available, and then to perform a safe surgery for you. Discuss your concerns with your board-certified plastic surgeon so with his/her guidance you can come to a decision you feel comfortable with. I wish you well.