I have had 6 instances where I haves seen a fish follow or come up to an un-scented lure and not eat it, then eat the same lure with scent. I think this has happened many more times, but these 6 times I kept an eye on that fish the whole time. 2 of those 6 times were with Trout-Plugs and 1 of those was a 7+# Bass. I usually used to think "I'm not going to bother with scent, it won't make a difference on this lure." Now I slather scent on everything, even Rat-L-Traps get scent now. I know it may be overkill, but it has made a difference.
On the opposite note, if you notice you aren't getting bites with a scent that you, would expect to get, don't be afraid to switch. Some scents get rancid after a couple hot summer days sitting in the car or go bad on the shelf at the tackle stores. I had a jar of Rainbow Trout Bait butter that after 6 months use and many good fish, finally went bad and not even a bullhead catfish in an aquarium would touch it. Others just aren't very good for 1 species or another. I have 2 bottles of BaitMate scents that have never caught a Bass between them, but I've used them for Crappie with fair success and in Saltwater with good success.
I wonder if the tube of Mega-Strike original scent that I got wasn't already bad. I haven't caught fish on it yet and I've given it a fair shake.
Tight lines,
Wade