Well, I'm a sucker for gadgets and tools and I don't regret any of the specialty rulers I've bought - except the duplicates! I've done that three times, now - ordered a ruler that I already have! I have a ton of rulers still in their wrappers and I'm not sorry I bought them even though I haven't used them yet - I might need them someday!
But, I've bought my fair share of klinkers.
The round or square "frames" that are alleged to hold your quilt firmly while you are free-motion quilting. They require constant picking up and moving and a lot of a pinching motion - RMI waiting to happen, except that you will give up on the darn things long before it has a chance of developing. They will be far more in your way than you can even imagine. I do use the round one to trace a large circle sometimes. Template plastic is a lot cheaper.
The ultimate pounce and screen stencils. No go.
I have blue and white and either of them can leave a visible mark on a piece of smooth fabric. A quilt is not smooth, though and holding the stencil down flat to the surface is necessary in order to get a transfer of the design. Once you do get the design stencilled on, you start quilting and guess what? The vibration of your sewing machine punching into your quilt rattles the miracle powder out of place in a 2-4" radius around the needle! LOL!
The first fabric tagging gun I bought was a frustrating piece of junk - and I can't remember what brand it was - it was the one sold in Joann Fabrics in the quilting section about five years ago. But the Avery tag gun made for fine fabrics (used with 1/4'' barbs) and the Micro Stitch guns are both great. Love, love, LOVE them!
Seams So Fast and The Angler have only been out of their packages once - more trouble than they're worth.
In my opinion, of course - other people have tried all these things and found them helpful.