Those are great questions, and many people have had similar concerns.  We have a resource on Synvisc and similar treatments that will give you a good overview.  Just click on this link:
Hyaluronan Injections for Osteoarthritis of the Knee

Synvisc and similar treatments (viscosupplementation) have been gaining in popularity over the past 10 years because the evidence has been supportive of their effectiveness for short-term pain relief (short-term would mean about 6 months or so).  We don't know for sure how effective they are in the long-term.  It's hard to say what the most effective treatment for osteoarthritis of the knee is for one individual.  That depends on how bad the arthritis is, and is something to discuss with your doctor individually (only your doctor can answer questions related to your husband's specific condition).

In general, initial treatment for osteoarthritis may start with activity modification and anti-inflammatory medications, among other treatments, and in severe arthritis where all other treatments have failed to provide relief, knee replacement surgery may be considered.  Viscosupplementation (which includes Synvisc and other similar brands) can be offered as another option during the overall treatment course for osteoarthritis.

Thank you for the insightful questions!