I never wear it. It just seems totally unnecessary to me and buying liners would take away from being able to buy more eyeshadow! So, I'll probably always pass on the lip liner... I don't even own one.
I like wearing lipliner because it make the lips appear more finished. I tend to wear hodgepodge or hover more than the others I have. Also I have a natural lipline that stands out with some sheerer colors, so I like to use a lip pencil to blend the line down a little so it's not such a huge difference. These two colors are perfect for that no matter what color I'm wearing.
I checked out the NYX liners at Ulta the other day and ended up getting 3 of them. They have a "Deep Purple" similar to Velvetella that looks great with dark lippies (I paired it with Odyssey), a "Natural" color that is good with nudes and several that are similar to the MAC shades at about 1/3 the price. I was happy with my haul.
The only MAC lipliner I have is Vino. I use it with Cunning lipstick from the new FF collection. I have to line my lips and fill them in because this lipstick seems to go on kind of blotchy otherwise.
I have other lipliners from NYX but to be honest, I don't use them. I only find them necessary with dark colours or reds - and even then I don't usually wear them. I'd rather spend my money on eye shadow.