do these things decompose or are they like most plastic in the world and have a half life of 500 years.
This happens to be an area where I have done some research...
Like most plastics, they do not decompose and are not degradable. They do break down into smaller particles of plastic (have you heard about the
Great Pacific Garbage Patch?). Like most plastics, they'll stick around for some time.
In addition to that, the PVC used is typically softened and made pliable using
phthalates which are linked to adverse health effects. FoodSource Lures, makers of biodegradable baits, have some links on this issue.
I can't help to question throwing plastics into my favorite body of water and thus I avoid plastics when an alternative is available.
I dabbled with bass fishing while researching products for Green Tackle and replacing the Senko was important to me since I've caught bass with Senko's in the past. I was able to catch a bass on a trip out to Henry Hagg (nothing to write home about... but caught one just the same) using a Food Stick from FoodSource Lures. The Food Stick is similar to Senko worms and I fished it exactly as you would a Senko (I rigged wacky with no weight). FoodSource lures are made out of real food--not plastic. They are 100% natural and biodegradable.
Another alternative for soft plastic bass worms, as was already pointed out, is Berkley Gulp! which is also biodegradable. The Turtle Back Worms come to mind for bass, though they have quite a few different options in the Gulp! line.
And finally, Rapala released Trigger-X baits which are also biodegradable. These are super high end and priced as such (we sell them for $6.29 for a pack of 3). But, they are supposed to be really, really effective and there are reports of them being banned from bass tournaments because of the pheromone scent they use.
The above mentioned lures are not likely to be cheaper than your standard soft plastics which are often mass-produced over seas. Being biodegradable, they will not be able to be re-used for years and years either. They need to be treated more like live bait. So the expense is a bit higher, however, you get better scent dispersion, fish hold on longer before ejecting the lure giving more time to set the hook, and you do good for the environment. So that's the pros/cons you'll need to consider.