Has anyone used these and are they worth it?

IF you want more power and they are for diesels then go for it. If for gassers, well they work but its harder to make real hp with a gasser.
If you want more mpg then buy one specifically made for it. Timing will improve your mpg but more fuel is only more go.

What vehicle to you have in mind?

1991 GMC Sierra Z-71 I am going to put a new 350 5.7l in it and trying to figure out what upgrades to put on it in order to help longevity and performance for towing my 20ft boat. Any ideas

Don't waste your money on a programmer/chip for a gasser. If anything get a cold air intake, some transmission upgrades, and new exhaust. In a gasser if you go for performance you're going to lose longevity. I'd shoot for the small things that are going to help you in the long run. Sure, you'll get a few HP out of the exhaust and intake, but they'll definitely help to keep your temps down which is the biggest enemy in longevity.

To improve longevity add a Fluidamper harmonic balance.

For more power & economy upgrade the ignition system with any MSD parts available. Add a good exhaust system. Edelbroch TES headers. See if Edelbrock makes a direct replacement intake manifold.

Also don't use a thick oil. Most engines require a 5w-? oil. The thicker the oil the harder the engine has to work to move it.

what is a Fluidamper harmonic balance?

What does MSD stand for?

MSD is a brand name. MSD ignition systems are some good stuff. Edelbrock shorty headers or the ones dave suggested are also a good upgrade and they're reasonably priced.

I could be wrong, but i'm pretty sure the harmonic balance is part of your serpintine system and makes it function a million times more efficently.

I have a Super Chip in my 07 Duramax. My fuel average since the install has steadily gone up. I have more power and use less fuel. At todays prices the Chip has already paid for itself. It was installed in January this year.

MSD=multiple spark discharge.A fluidampner harmonic balancer is usually used in high performance, high RPM motors.I don't really think it will give you an advantage on your truck.

He has a gasser, not a diesel.

http://www.fluidampr.com/

All engines have a harmonic balancer of some type. The better they are the less internal vibrations. This can result in more power & longer engine life.

hope none of yall take your warranty vehicles in for service with a chip in it.. they WILL void your warranty. take it out and put it back to factory specs before you take it in....

it appears that fluidampers are only for diesel, is that right?

No. They make them for several applications.
620101 is the part # you should need. I would contact them to be sure 1st.

http://www.fluidampr.com/CHEVROLET.htm

Watch your transmission, I had a Z71 like yours way back when. The engine was stout but I was having some tranny problems around the 90k mark. She would over heat bad even with the cooler installed. Talked to my transmission mech and he told me it was a design flaw which they have corrected. I did install a higher output ignition system and Flowmaster exhaust on the truck. On the hwy dry I could get 20-21mpg.