Darvocet has been on the market since 1957 and I can't find any warnings of negative long-term effects as long as it is taken as prescribed.  Of couse, taking more than the prescribed dosage or drinking alcohol while taking it can result is serious consequences including liver damage (from the acetaminophen), respiratory depression, and fatal overdose.  Also, be careful not to take any other medications, whether prescription or over-the-counter, that contain acetaminophen.  

If you've been taking Darvocet for many years, you have a physical dependence on it.  If you ever decide to quit taking it, you would need to consult your doctor and taper off gradually to avoid withdrawal symptoms.