I have had the same issues. I have now bought two Deltas, thinking the first was defective. After less than a days use, I have leaking and gurgling issues. I haven't noticed any obvious deformities in the silicone seal other people have mentioned. I have gone through at least five coils, and all leak. I have e-mailed Joyetech, which has been a painful process. They would basically e-mail back with questions, such as: have you cleaned it etc. Finally, last email I received said I need to take pictures of the device to start a RMA process.
When it works, this device is fantastic. Great flavor and clouds. But I find myself now cleaning this thing once to twice a day. I get liquid under where the coil screws it (the chamber with the battery contact and air holes). Liquid also gets above where the coil screws into the reservoir. I have tried high wattage (up to 20), pulling hard, pulling softly...none of it matters.
I NEVER let it get below 10%, in fact, I usually keep it above 50% because that seems to be where it starts to leak quickly.
The store I bought it from was totally unhelpful. The guy who runs the place has the same device and says he has never had leaking issues.
I use a Five Pawns 50/50 liquid. Nothing crazy here.
I suspect something has changed in the manufacturing process recently. Perhaps the seals? Either way, I would not recommend anyone buy this product at this time. Hopefully Joyetech gets its stuff in order and fixes the problem. Until then, I'm going back to my Aerotanks. The clouds may not be as big, but I rarely have to whip out the Qtips for old faithful.
EDIT: I have completed their RMA form. I will keep this thread updated with what happens.