Both pens are really easy to apply thanks to the fine tipped flexible nib. The black pen has replaced my liquid and gel eyeliner as it's so quick to apply. I use it daily; just a sweep across the lids, a flick at the edge and I'm done.
The quality of the pens differs between the two colours. The black pen isn't as strongly pigmented as I would like - I have apply heavily to get an opaque colour. The staying power of the black pen is also lacking. For everyday wear to my lectures it does the job but I'll swap it for my Maybelline Gel liner for a night out or an 8 hour day.
Unlike the black pen, the teal has massive staying power, it's almost semi-permanent. It's pretty hard to get off without leaving faint green lines. The teal pen is is also more pigmented, and it feels a wetter to apply than the black. The problem with this pen is that it transfers onto my upper lid and no amount of priming or powdering my lids overcomes this. This is such a shame because the colour looks beautiful on, but I can't wear it because I'm constantly worrying about getting green lines in my creases.
As for the 24h hour staying power, I haven't tested them for 24 hours but I definitely don't think that the black shade would last that long, in fact it would be unlikely to last just 8 hours. The teal on the other hand might last you though the night, but would probably leave you with some 60's style green lines as well.
Overall, I do like these pens. The black is my go to daily eye liner, although the formula could be improved, it definitely does the job for me, it's really easy to apply, and at £2.99 it doesn't burn a hole in my pocket.