I bought their red ink in a bottle, assorted color cartridges in international and Lamy form, and red in international cartridges. These came from pens_and_more on eBay. There's lots more still available. Their prices are low but shipping for individual bottles is $2.95. They do have some very low prices on cartridges if you buy enough that there is no charge for shipping. Likewise if you want many bottles of the same color, especially if it is black or blue, but, alas, no assortment of colors in bottle. My main impression is that the ink is inconsistent. There are lots of reviews on Amazon that seem to bear this out.
The assorted cartridges are opaque and the color of the ink, while the cartridges in a single color are transucent. The swatches on eBay look very dark with black overtones, somewhat like Noodler's Black Swan in Australian Roses. In no case did the ink match the swatches in this regard.
For red, the ink in the translucent cartridges is deep and rich, like Waterman but more so. The ink in the bottle is a very generic medium red. Flow is erraric but generally the reds were hard to start and restart. I liked the red in the tanslucent cartidges and ordered more, but expect to have to be careful what pen I use it in.
Orange is a pleasant color but a bit light to read. Green is also a bit pale, and with a bluish tinge like many European greens. Turquoise and purple similarly look a bit pale and European. These two are a bit darker in the short, opaque international cartridges than in the Lamy cartridges. The purple is on the blue side. Blue is a bit murky. Black is dark grey, with the opaque short international cartidge paler than the Lamy. The brown is quite reddish, almost burgundy. Pink is quite legible for a pink ink and I would choose this over most other pinks were I keen on writing in pink. The yellow is totally illegible for handwriting. Was it meant for highlighting?
About half my Parker pens will take Lamy cartidges, so I gave that a try. The usual problem is that the Lamys are a bit too fat to slide into the section. That was true here, but I had the additional problem that very few of the Parker pens could punch a hole in these cartridges. Not having any Lamy pens on hand to use them in, I noticed that the other end of these cartridges looked suspiciously like the end of an international cartidge, so I tried some upside down in pens that take international cartidges. About half the time the cartidge was too long to allow the pen to fit back together,but when it was not, these Lamy cartridges worked OK as very long international cartridges. I seem to recall that Levenger's long cartridges once functioned like this.
Many of the test pens using the opaque cartridges seemed to be out of ink within days and all I wrote with most of them was a few lines of test text. I could not tell if there was not much ink in there to begin with, or whether the ink had dried out or clogged.
All in all, only the red (in cartridges), pink, and brown were unusual enough to keep. The rest had the advantage of being cheap at the expense of being erratic in performance (see also Amazon). Someone wondered if these were the same inks currently sold under the Sheaffer brand. I have never had any trouble with that ink, and wonder if Thornton ink is from batches that did not pass Sheaffer's quality control standards.
Thank you for that thoughtful and informative review. I didn't try the single color ones so that was interesting to find out. I also have not opened my bottles.