The cost of a reexamination proceeding to a third-party requester may be a small fraction of the cost of litigation. A reexamination may also provide for a quicker determination of the effects of the prior art. In addition, a third-party may request reexamination of a patent without engaging in any allegedly or potentially infringing commercial activity. This allows a company to test the validity of a patent's claims before expending substantial resources to develop a product or process that might later be found to infringe.

However, reexaminations have significant limitations:

• some issues of patentability that can be considered in litigation, such as the adequacy of a patent's written description, compliance with the "best mode" requirement, or bars arising from prior public use or commercialization cannot be considered in a reexamination;
• a reexamination is limited to considering documentary prior art (patents and printed publications);
• unlike court proceedings, reexamination does not allow for subpoenas, interrogatories, depositions, or live testimony and cross-examination of witnesses;
• an inter partes reexamination can create an "estoppel," preventing the requesting party from raising arguments in litigation that could have been raised in the reexamination.

A reexamination that does not invalidate or significantly narrow the claims of a patent can leave the patent stronger than before. Thus, it should be used only in limited circumstances.

http://www.wilmerhale.com/publications/whPubsDetail.aspx?publication=392