This eyeliner is really easy to apply due to its thin brush tip and it stays on pretty well. After several uses, though, the eyeliner color doesn't seem to apply as vibrantly as it did before. Requires vigorous shaking before use. Also, the container size is rather small so I wound up going through the entire product rather quickly. I would attribute that to the product itself drying out in the container a lot faster than other liquid eyeliners I have used. Still, the product stays on my eyelids all day, so I approve!
I had the same problem as gabbymt. I loved the felt tip, but because it wasn't waterproof, i found that the product was a bit runny. However, recently I found a better product by e.l.f. that is a waterproof felt-tip applicator. I'm not sure what it's called but you can find it on eyeslipsface.com.
This eyeliner didn't work too well for me. It kept running under my eyes after a few hours. I was constantly having to look in the mirror to fix it and not embarass myself in front of my co-workers.
I have been using this eyeliner for years and wouldn't be tempted to trade. Although it is not waterproof, it is long-lasting. My favorite feature is actually that the applicator is more of a felt tip than a brush, allowing for a lot of control when applying the makeup.