Fantastic, gorgeous historic style and detail. Here I compare the marking system to others. Most visible: Ted Lesley DIY, with blocky letters; 2nd, Bicycle's PRICEY UMD. I find the heart icons too tiny, and hard to spot, yet the letters are quite clear and overall they’re the most readable common deck (but that price – ouch, and I’d worry about the marks being spotted).
Next group, easy to read, although a bit less so than UMD. Subtle and harder to see for spectators: Cohort Ghost, followed by Pioneer. They are both excellent systems, reasonably readable, but well hidden, and the Pioneers moreso, in particular their C and D icons (which are almost too similar but with a bit of use I think they’ll be fine). Then come the 1900’s, a radical break in method. They require a few hours of training to read but they are diabolically designed, undetectable, cryptic, and beautifully weathered. Love ‘em.
So, all in all Pioneer does VERY well in my lineup – not quite as quickly legible as UMD, nearer to Ghost, very hard for others to spot, still readable, and a gorgeous historical art deck, a real winner. I’d use them when I’ll have an extra second to look; for a faster reading situation, I’d go with Ghost Cohort. For something bold and easy to read in a flash, UMD’s best. For historical style and retro art, Pioneer wins. For ghostly cards for Halloween with a tale about how they were kept locked in a chest due to being cursed, followed by a ghost card act or two, 1900s would be killers (and the 1800s, although I got the old corner marked ones and had to re-mark them).
And for a gorgeous deck to just play cards with, the Pioneers get top marks!

I didn’t happen to buy a Pioneers deck from Penguin, but I liked the one I wound up with so much that I’m purchasing a couple more here at the shop.

I gave it 4 stars only because I don’t want to mislead anyone: it’s not the greatest marked deck in the world. Really the fact that it’s a serviceable marked deck (not the easiest to read, but certainly not the hardest) is secondary to its handling. These cards feel amazing—smooth sliders when you need them to be but ready to stay put, too. Pioneers are a treat to practice with and I will likely start performing with them as well. They leave the same company’s Cohorts line in the dust for sheer elegance (lovely back design, equally lovely court and number card tweaks) and handling. The blue deck even comes with a red/blue double backer. Try a deck and see if you don’t agree that it’s worth it to have at least one lying around to play with.