I've had great results with both their SW and BM samples. I haven't tested out a bunch of them, but the few I did end up actually painting were pretty spot on. i haven't tried the Farrow and Ball samples personally, so can't speak to those

Samplize actually uses real paint from the manufacturers for each color and applies two coats of it to each sample.

The only issues is if you buy a different sheen from the eggshell Samplize samples, it will look slightly different in flat, matte or semi-gloss.

Green looks different in different lighting.

They are going to be reasonably accurate. The use actual paint to spray their samples. Will the tinting machines they use be calibrated exactly like the tinting machine your store is using and will both be using tints frim the same lot - probably not, but they are as close as if you purchased two gallons from two different SW stores.

They are also flat samples. Texture can influence the appearance of color, so if you are painting orange peel walls the color may look different than it would on a non textured wall. Gloss levels also influence the appearance of color. Their samples are eggshell.

I purchased a few Samplize for SW colors and also purchased the smaller sized ones (8x11) at a SW store. Samplize arrived folded and I was never able to get the creases out. I used to buy small pots of paint -- they don't seem to sell those anymore :(

My painter instisted that the primer was untinted ... but it was slightly beige, compared to the whitest white I sampled.

Ive ordered many sample size to match a particular paint formula. Only 1 time did it not match. We painted on the sample size and it was clearly off. The paint store employee said “this happens some times”.

He then fed the sample size through the machine to color match to the sample size and tweaked the formula. He painted that onto the sample size and it was identical. I would advise start with 1 gallon or quart
and test it out on the sample size and wall. As others said, your room lighting will also alter the look of the color. Hope this helps.