Had the Delta 2 since I was one of the few people that liked the Delta 1 (it leaked and had a quite restrictive pull, but worked well otherwise and the coils lasted very long (at least 3 weeks with about 5 ml of daily consumption!)), but was not satisfied. The ceramic fibre burned up and got in the middle of the vertical coil (never exceeded 30W and primed them carefully). Now I tend to think that I have not pulled hard enough.
Got me the ST Mini instead. No problems so far: No leaking, no dry hits, very good taste (from the very first pull!), lower energy consumption (2200mAh last nearly 1,5 days at 18.5W), the tank is not too small or too big, coils last about 10 days (with 10 ml a day), many options (0.5, 1.2 or RBA), many goodies included (second tank, second pair of black O-rings and so on), less noisy than the D2, you can see the liquid and disassemble the tank... I could go on like this forever ^^
But: It is a direct lung hit only device. Mouth to lung does not work with the Subtanks (imho). The airflow is too airy for MTL and I tend to cough when I try (vaping since 2.5 years, so it is not the typical "newbie cough" that you get used to).
EDIT: @djsvapour: The Delta 1 coils lasted very long. With the Delta 2 I had to change the coil every 2-3 days, But as I said: Maybe I did not pull hard enough.