I've got a Cthulhu MTL, and really like it. I agree with the others, wicking is key to whether or not it leaks. First few times I wicked it it was amazing with no leaks, and has probably the best flavour out of all my tanks. It's quite well made, and even though it's short, it holds 3mm of juice (it's a 23mm tank).
If you wick it wrong... it's going to leak out of the airflow, which happened to me a few times. It'll also gurgle due to wicking/filling styles, but it's possible for it to _not_ gurgle, as I've managed that a few times.
The black drip tip (smaller diameter than the red) sucks up juice from _all_ my tanks and deposits it in my mouth, so I don't use it. I find this is the case with many small-bored drip tips, however. I love the provided red one though, and have no complaints.
Cthulhu has airflow inserts, but I don't know that they're really the answer to having a good MTL experience (which isn't a criticism of Cthulhu Mods - lots of people are bringing out restrictors to "enhance" MTL) - I've got the middle sized restrictor in there, and although it's a tighter draw, I don't think it's really making the vape better. Still, I guess they serve a purpose
I have a couple of Siren 25s, and like them a lot too, and will chase down a 22mm one so I can fit it on my Pico mod. Don't have any experience with the Merlin.
TL; DR
I really like the Cthulhu MTL, and feel wicking it correctly gives you a great, not leaky vape. It's cheap, solid, and looks great.