From my experience, and I have done some testing on this, it is worth noting that all tests have been completed in victoria (mainly ppb and wp and all land based, land based because I don't know anyone else who has a yak to do the tests with!). Some of the scents actually repel or have on effect on the fish. This, I found to be a common trait for most foreign brands. The gulp sprays seem to have a small effect but only when the fish were not very interested. By far the most success that I have had is with the s-factor. When the fish are a little timid or not biting at all, the lure with the s-factor definitely sees a lot more action. The species I have intentionally experimented with are snapper, squid, flathead and silver trevally. The last two times that I have been targeting the silver trevally with a mate to test this theory I have come up trumps (me with the x-factor, and my friend without) catching a total of 6 between 34 and 53cm, while he caught one at 29cm. We were both using the same line/leader class (6lb main + 10lb leader), with very similar rods and reels and the same lures.
I have had some positive results with the snapper or rather pinkies but plan to reslly test this this fast approaching season. The squid however have bee very positive towards the extra juice. For example I went to Mornington pier thursday night, was told by 4 different people that they were not on the chew and the 3 or 4 hours that they had been there had resulted in nothing. My girl and I ended up with 6 in 2 hours using a small amount of s-factor, and the gulp spray (both seem to work well with the squid). I managed 4 using the x-factor (one was about 1-1.3kg) and my girl 2 using the gulp stimulate spray (she prefers the spray so she doesn't have to get her hands dirty

). No one else caught anything while we were there and from those that I asked, none were using any extra scent. These are the two most recent attempts that I have conducted in testing this extra scent theory. You can argue that it was not the scent but rather the type of lure or squid jig mixed in with angling skill or retrieval action and a bit of luck. I am however convinced that in some cases, especially when the fish are quiet, that it makes a world of difference.
Sorry I didn't really stick to the true topic, I have made a few of my own scents by using flour, water, butter, aniseed and a tiny bit of tuna oil, don't really think it worked well on the salt water species, but while using bread to try catch trout, If I had some of this scent on the line and covering the bread it did increase the catch rate.
Cheers
Lobo Loco