The only downside to this pen is that it runs out, or rather dries out very quickly. The only way around this (to avoid wasting money on an eyeliner that you only can use for a few days) is to leave it for a few days (perhaps even upside down) and the pen nib will be filled with ink again. Due to this, I have a few Liner Feutres lying around my room to be picked up at various times!
All in all, I would say that this is quite a good buy, it costs more than the average liquid eyeliner and can be frustrating when it runs out, but the actual result of the eyeliner is very nice!
For videos on how to apply eyeliner (and other tips!), go to Bourjois' YouTube channel for some fairly simple ideas.