I've used a lot of pearls and other products from them, as well as others like HOK, more in smaller projects (ie bicycles) than full repaints... very often mixed into SPI intercoat clear (with a little hardener in it) with great results. I strongly recommend playing a bit with what you're thinking about on some test coupons as a little pearl powder or concentrate goes a long ways and the effects of how much pearl, how many coats, etc... can be somewhat unpredictable (to me) especially when playing with multiple layers... Also, in my experience sometimes simple and less is more when it comes to things like this. A few things that i really like the way they came out...
my truck is black (ppg-9700) with a fair amount of silver sparkle pearl added and the last two coats of color cut with a significant amount of intercoat clear, and a little gold ghost pearl added to the last mist coat... it looks deep black when you look at it, but then the light hits it right, it sparkles a but and has a slight gold halo... subtle but kind of cool...
one of my daughters bikes (bicycle), yellow "sparkle" pearl with a little tangerine pearl in spi intercoat over bright white base, then dusted with couple light coats of gold ghost pearl in intercoat before 4-5 coats of clear (I like the harder, more durable Euro than universal for stuff like this...). it came out a really bright yellow with an orangish tinge and almost looks like it glows when light hits it...
I also repaired the metal flake striping using the right size flake mixed with SPI clear on one of my boat projects (black with silver flake highlights...)... and then the whole thing with SPI clear (after wetsanding al the oxidized gelcoat off)... the first couple coats with a little "sparkle" (large grain) silver pearl... It came out fantastic in my opinion and is holding up great to heavy use years later.
I know from reading on here a lot more than I post, that there are some folks on here who do some seriously beautiful professional work. I'm a backyard hack at best, but have fun doing it and have had some really nice results... I have greatly appreciated the generally helpful attitude on here and the wonderful experience shared by some of the truly talented / experienced members (not to mention Barry the chemist...).
oh, if you use the cheaper powder form stuff, i've found that it's important to mix very thoroughly (I do before adding catalyst / hardener) and to filter after letting it sit for a bit... Again, no expert, just my experience...