I wouldn't call it a gift from the heavens, but for a beginner reloader with a empty spot in the turret that may not have the dies set up perfectly, its cheap insurance IMO.
With that said, I use it as a dedicated crimp for 44mag, and don't use it on anything else.
I never really saw the need for most calibers. I don't crimp most rifle ammo at all, including for the M1, M1a, and AR.
Some specialty calibers like the .357 sig, that collet style Lee factory crimp die was the only way I ever was able to make dependable ammo. The .22 Hornet with it's paper thin brass is another one. I also like it on cast lead bullets for the 30/30.
I use the FCDs with most of the handgun rounds I handload and find it takes away any concern of the reloaded round not chambering (have had rare issues with this and some handloads before starting with the FCDs). I got into them seriously when I started wanting really precise control of seating and crimping, back when I was really hotrodding handloads. I like to tell myself that the rounds resulting from the use of the factory crimp die are better, but I suspect they're not so much better that it matters. I like them in my rifle handloading, as I appreciate the extra control they provide in the crimping.
Hey, do what ya wanta - it's a hobby and it's America.
Whenever a new "rifle style" (collet) Lee FCD becomes available in a caliber that I load, I buy it immediately. By seating and crimping in two separate operations I get more consistent OAL and no shearing of lead at the case mouth when using lead bullets (regardless of crimp groove style - some designs suck for seating and crimping in one operation). By backing the FCD in/out, you can crimp heavy or light without affecting the adjustment on your seater die. Conversely, when switching between two bullets with similar, but different profiles, I can adjust the seating depth to match the crimp groove without messing with my crimping operation. Adjustments to either operation become insanely easy and I can achieve EXACTLY what I want with ZERO frustration or damaged bullets. In some handgun calibers like .357 Mag, .44 Mag, and .45 Colt, Lee makes two styles of FCD. I use the "rifle style" collet FCDs for these handgun calibers and am very pleased with the versatility and ease of changing my setup - these dies are worth every penny. For rifle calibers, the FCDs are useful for the same reasons and for peace of mind, I crimp anything that's going to get fed from any type of magazine. FWIW, some single shot rifle and handgun loads using lead bullets and small charges of fast powders can be more accurate/consistent with no crimp.
I only have one, but I really don't know what they are supposed to do that another crimp die wouldn't? Supposed to crimp with horizontal force rather than vertical right? I have one for 458 SOCOM. I decided to use it instead of using my Redding seat/crimp die to seat and crimp in one step. The biggest factor in my decision was that I paid something like $12 for it. It is different than the Lee FCD dies I find looking around online because it doesn't have a stem for adjustment. It requires the shell plate to compress the mechanism at the bottom of the die up and into the die body to squeeze the case. It works and the price was right. I think I prefer my Redding micrometer crimp dies for the other calibers I load, but they were more than $12!
I had never sprung for one until early last year, when I assembled 1K .45ACP loads for a friend, with him paying for the dies and all required cartridge components.
I bought the Lee four die setup for the project that includes the FCD, and I found out just why this particular type of crimp is so useful, lat least for straight walled cartridges.
For one, unlike the conventional crimpers, it uses a collet, so it's much easier to find a uniform level of crimp.
Also, it's especially nice that it also runs the entire cartridge up through a secondary carbide ring to ensure that the case doesn't get bulged enough to hinder chambering.
I've since bought a FCD for my own two straight walled cartridges, the .45 Colt and .38 Special.
Other than for cast bullets or cartridges with abnormally thin case neck walls or short necks (the aforementioned .22 Hornet or .357 Sig), the utility for bottleneck rounds seems more debatable.
I like it because I can get a good crimp on a bullet without a crimp groove or a bullet with a crimp groove I a location other than where I want to seat the bullet.
I don't like them personally. I shoot only coated and cast bullets. For best accuracy, these bullets need to be at or a bit larger than bore diameter. I found that the FCD swaged down handgun bullets in 38, 40, and 45 resulting in leading and poor accuracy. Have had best results using a taper crimp or a Redding profile crimp die, and putting on just enough crimp to take the bell out, except for 357 mag, where I use a heavier crimp to make the bullets stay put.
YMMV.....
edit: the collet style die looks interesting... I may give that a try. My comments were on the factory crimp die which sizes the whole case.