All silicone breast implants are made with polydimethylsiloxane molecules cross-linked to each other. The fewer crosslinks, the more liquid (oily) the silicone is, and more crosslinking yields more and more cohesive silicone gels. The present fifth generation of silicone gel implants (Memory Gel by Mentor, or Natrelle by Allergan) are both cohesive gels that do not leak or exhibit "silicone bleed" from the implant shell beyond microscopic molecular amounts that are substantially less than in previous generations of silicone gel implants (these were the ones that could rupture, leak, or stimulate inflammatory reaction and capsular contracture.) The present implants were engineered by both implant manufacturers to address and deal with these issues, and are much more cohesive than previous versions.
The "gummy bear" implant is an even more cohesive (more crosslinking), form-stable, teardrop-shaped, textured-surface implant that has been used in several studies that are hoped to lead to FDA approval. They are not yet available to the general population or plastic surgeons outside these studies (which I believe are presently closed). There are pros and cons to these implants that your surgeon can discuss.
At this point in time, both the Mentor Memory Gel and Allergan Natrelle silicone gel implants represent the best FDA-approved implants available, and are comprised of a soft but cohesive gel that does not ooze or leak if these implants are cut or punctured. They are NOT, however, as firm and form-stable as the shaped "gummy bear" study implants that have not yet received FDA approval.
No implant (or any implantable device) is guaranteed to "last a lifetime." However, silicone gel breast implants do not "wear out," nor do they automatically need replacement after any set period of time. The present generation of cohesive silicone gel implants will likely outlast your own anatomic changes or desire for different size or position, which are the most common reasons for re-operation, NOT implant "failure!"