Is the Last Record Preservative system a worthwhile investment?

Yet another happy user since 1982 here.  I can't say that it improves the sound quality, but it does drop surface noise near to the limit of whatever the vinyl material can deliver.  That might seem like it improves the sound simply because the S/N ratio is maximized.

I have found LAST works quite well on all the vinyl I've purchased pressed from ~1955 to the present.  As others have noted, the key is starting with a clean, dry surface.  If the vinyl is new, I use the LAST Power Cleaner (mold release remover) first, then I use an ultrasonic cleaner with distilled water and a splash of LAST machine cleaner solution for everything, new or used.  Another wet clean with LAST cleaning solution and my trusty Discwasher brush, a thorough dry and then the surface is ready for preservative.  If you've gotten everything right, you can actually feel the preservative bonding with the vinyl while applying it; the brush seems to move with less friction. I'm sure that's due to the molecular effects visible in the micrographs referenced above.

FWIW, I've also used Stylast and the LAST stylus cleaner since they became available.  I've never identified any audible difference, but I can say I've never worn out a stylus.  The magnets have gone south first.

From my perspective, a good value proposition for a modest investment in happy listening.

I decided to use Lyra stylus cleaner instead of LAST because they claim it does not dissolve the glue, and it doesn't.
Besides, I have $500 MM cartridge and don't worry about stylus wear, it's $375 to replace. For now, after about 700 hours of play it is not far from new.

If any who haven't tried Last lives in or near West Palm Beach, Florida, I would be happy to have you bring a record or two over to the house and I will treat them for you.  You can listen before and after treatment and decide for yourself if you hear a difference.  My system is state of the art and will reveal all. 

By the way, tape also sounds a little better when the play head is treated with head preservative, and record head when recording. Tape moves more smoothly, very simple. I don't use LAST tape preservative because I can't apply it with cassettes, with open reel deck you can.

My experience with Last initially was not good. I found it really deadened the sound, the dynamics of the recordings. It seemed to make them a lot darker. I was cleaning the older records I had purchased more recently and that had surface noise. I used the GEM Dandy record cleaner and then followed with ultrasonic bath and then Groove Lube, which must be used very sparingly and according to instructions. Then, at the end, I applied the Last. I found the initial results quite disappointing to say the least. The records were, however, much quieter and the surface noise was pretty much gone. The Groove Lube left a bit of gunk on the stylus but I removed that easily enough on each side with the Magic Sponge that's been recommended here many times for stylus cleaning. I got busy and then came back a few weeks later and listened to other LPs I had cleaned with this method. Well, after waiting several weeks, the Last preservative was no longer imparting any kind of unwanted sonic effects. The sound had changed completely back to normal. I still have not applied it to the brand new records I've purchased, but I will say that I will continue to use it for older LPs and I might try it on new ones after I've had more experience with it. So I would recommend it then so long as one gives it enough time to fully settle in.