In the first picture I have swatched the Clinique gel liner in 02 True Black and on the right is MAC in Blacktrack. Both are obviously black gel liners. I have had the Clinique one forever. It's almost gone and what is left is definitely starting to dry up. They both look the same swatched and I was heavy handed with the swatches so please forgive me. Both took me three cotton balls of makeup remover to get off.
They are both solid products. I would happily buy either of them again. I have to say though that in my humble opinion, the Clinique one is a bit superior. It is creamier, smoother and in general easier to work with and more forgiving if you make a mistake than the MAC one. I believe they are around the same price - around $21 - I think.The Clinique is in a plastic, cheaper container and the MAC is in glass. Props to MAC. Products always do better in glass whether we are talking about food or makeup.
I also find that the Clinique one stays true all day and maintains the same black intensity. The MAC one stays pretty true but just looks drier and a bit more powdery on I find. If I had to choose between the two, I would buy the Clinique one again first. The Clinique one also comes with a liner brush which looks cheap as all hell but is surprisingly easy to use and works well. I have heard that the MAC one dries out faster than a lot of gel liners but I haven't used much of mine so time will tell. A good tip I hear is to store them upside to keep them from drying out. There's your tip of the week.
I reach for the Clinique one most of the time when I want a black gel liner. I guess that says it all.
Rating: 9/10 Clinique (lost a point for plastic packaging and narrow jar opening)
Rating: 8/10 MAC