I've been using the Siren 2 for a few months now, and I couldn't be happier.
The juice I'm using is around 50/50, because winter is coming.
For the coils I'm using Kanthal A1 26GA (0.4mm i think) , 2.5mm ID, 7 or 8 spaced wraps, around 1 ohm. All this with Cotton Bacon Prime tight wicking.
The only problem that I had after a few weeks was that the base was getting stuck and it was very hard to unscrew (usually i managed to open it with a cloth over), recently I started "coating" everything in liquid so that the parts don't get so stiff.
There was a period also when the cap would seem to open too easily, felt like the silicon seal was losing its properties, but after a good wash it was as good as new.
Sometimes I'm getting weird strong throat hits after setting up a new coil, but I'm sure that's my fault, sometimes I forget to dryburn the coil or I place it too high.