There are obviously pros and cons of all three types of ACL grafts. First, though, I'll mention the hamstrings muscle group consists of three muscles as gotwood noted. The Biceps Femoris, the Semitendinosus, and the Semimembranosus. Oftentimes, they harvest a slice of the Semitendinosus. This is okay because it will regenerate to some extent. Also, this muscle's actions are shared with all three hamstring muscle as well as the Sartorius and the Gracilis muscles to a degree. The Sartorius, Gracilis, and Semitendinosus share a common tendon and insert on what's called the pes anserine which is found on the medial side of the proximal tibia.
Patellar Tendon
Pro
-Bone-tendon-bone graft often "takes" better due to the bone that is inserted into a hole essentially.
Cons
-Pre-patellar pain that normally lasts for a LONG time. Often something that will be nagging for the rest of your life.
-Patellar tendon weakness
-Scar
-Single strand
-Additional "injury"
Semitendinosus Tendon
Pro
-Double strand
-Less pain
Cons
-Potential semitendinosus weakness
-Additional "injury"
Cadaver graft
Pros
-Normally an actual ACL
-No additional "injury"
Cons
-Risk of rejection
-Risk of infection
-Seems to be higher risk of re-rupture
-Some people think it's gross
This is just a brief outline of the different types of grafts.
John, keep us updated. Hopefully surgery goes well in the morning and he can get back on track to recovery ASAP! Ask away if you've got questions...