Marvy Le Pens are fairly cheap, I think each pen cost under $2, and the construction reflects that. Le Pens have a plastic nib, and feel light in the hand, especially when compared to more substantial pens. The Marvy website doesn't specifically recommend these pens for drawing or sketching, so I can't penalize them for not holding up against technical pens like Sakura Microns or Copic Multiliners. These pens are perfectly fine for writing and doodling, and have decent ink flow.
As you can see, the nib is pretty tiny. When inking over non photo blue lead, the ink picks up a bit of the lead, but not enough to cause staining. You can still see the lead beneath the inks, but it's not unattractive. Inking over graphite might permanently damage the nib, however.
I gotta be honest, I don't get how people can do dynamic lineart with technical and finepoint pens- it's so labor intensive and there are often better options available. When it comes to colored lineart, the Marvy LePen selection is pretty extensive with 18 colors to choose from including a pink so hot it doesn't photograph properly (it looks downright neon in these photos!) I don't know how well Le Pens scan or reproduce, however, so what you see in these photos may not represent what they're capable of.
Marvy Le Pens play decently well with Signo white gel pens, and aren't a bad pick if you're looking to add a little pop of color to your work.
Fude Pen Test
If you immediately try to color with the Le Pens, they'll smear the fude, but if you wait a couple days, they're fude-safe, making them a good choice to add small pops of color to your sketches.
If you already like inking with technical pens, Marvy LePens are a fun addition to your inking arsenal. If you're like me, and vastly prefer the flexibility of fude pens, Marvy Le Pens make great writing tools, fine doodling tools, but aren't really a viable option for sketching. They come in a wide range of fun, bright colors, and are a fantastic way to liven up your planners and notebooks.