I'm a little late on the 'Gel Eyeliner' train but, i'm on it! Mac Fluidline is supposed to be one of the best gel liners out there. They come in many colors, I got the basic black one to start with.
'Blacktrack' comes in a little glass pot. It is not at all heavy, pretty light weight for a glass pot. As all gel liners, it is thick, creamy, smudge proof and very very black. If you are someone with oily eyelids than this ones totally right for you. For me, I like a eyeliner which stays for a long time. This one was perfect. It stayed on throughout the day and it didn't smudge one bit! The texture is like melted butter. It glides on the skin really smoothy and it is not greasy or heavy.
The color is very black as you can see below. It has a matte effect. One swipe is pigmented enough, you don't have to go over it to make it darker. But if you want you can go over it with a black eyeshadow to make it darker and soften the edges.
It doesn't come off easily. So you will need a makeup remover to get it off. One important thing is that you must close the product as soon as you finish using it or else it will dry. The Mac makeup artist also told me that you should keep the product upside down so that it doesn't dry out quickly. I didn't know that before. The only downfall is that it doesn't come with a brush. You will have to buy that separately.
It definitely is pricey for the amount you get but if you think about its merits - thick, smudge proof, matte black, easy to use, its a small price you have to pay. 4/5.
Take a look at my last post on how I do my Winged Eyeliner.
PS - You can use this on your waterline too! :D