what are the chances of overturning a dui if your blood alcohol level is .08?

There is always a chance.  Perhaps the test was not administered correctly?  Perhaps the device was faulty so as to give an incorrect reading?  Additionally, this is a highly technical area of the law and procedural impropriety's can occur that would result in having the charge dismissed (or at least reduced).

A DUI triggers both a civil and criminal case; it is always worth fighting it.  What you need to do now is to consult with an attorney experienced in these type cases.  Find one that practices locally to where this alleged incident happened.  They will have contacts with both the prosecutors' office and the court, and can utilize those contacts to your best advantage.  You don't want something like this on your record if you can help it.

Although I do not practice law in the State of California, here are my initial impressions.  First, although I generally agree with my colleague's comments, I'd like an opportunity to add an additional perspective.  The general rule is that there is a certain degree of uncertainty with respect to the test that was administered.  Assuming that the legal limit is .08, that degree of uncertainty could be used to negate proof, beyond a reasonable doubt, that your BAC was over.08.  However, it will be necessary to hire an attorney, and that attorney will probably have to hire an expert to testify with respect to the test's uncertainty, in order to properly utilize this defense.  Nevertheless, the fact that your BAC was so close to the legal limit may very well provide your attorney with an effective defense.  Good luck.