FWIW, here are some points I have learned:
Always shake your juices bottles. Especially first drip/fill of the day. Some juices are more complex and CAN separate. While I do not shake every drip/fill, I do it each time I change flavors.
Burnt taste - it's often all about the heat:
Many of us have read about watts (heat). Some of you will already know this so I am re-posting for those who may not.
Vg can burn at lower temps than PG. So higher VG percentages in your juice may be more sensitive to that burnt taste if you are not keeping an eye on the watts (heat). The following math (yea - sorry) is a generalized guideline.
Watts can generally be calculated pretty easily. There ARE other factors (volts under load, various battery levels, etc.) but for this purpose, lets just get the general idea.
Volts squared and then divide that number by the ohms of the atty = watts (heat) -- many people seem to like watts in the 8 to 10 watts range with most saying 8 watts is ideal. Some prefer hotter (risks that burnt taste) and some like it cooler.
So, in formula format: (V x V)/Ohms=watts
Most regular 510 attys are approximately 2.5 ohms There ARE other versions: LR is usually between 1.5 to 2.0 ohms, there are 3.0 ohms and 3.3 ohms. There are also High Voltage 510 attys (HV attys) that are either 4.5 ohms or 5.2 ohms.
Any atty can vary up to +/- .3 ohms
901s are native 3.3 ohms but also come in LR and HV varieties (usually 1.8 and 5.0 respectively)
Most 306 attys are LR and in the 1.5 to 1.8 range
801 series (801, BE112 and 302) are usually between 3.1 and 3.3 but there are also LR and HV varieties of some of these.
Cartos can really vary, again with LR versions various regular versions and HV versions. I am not a carto user but your vendor SHOULD be able to tell you the ohms of your cartos. Or, you can buy a multimeter (some as cheap as $5 or $10 and you can easily meter read any atty or carto as well as regularly montior your batts = good idea especially with HV batts.
Each model atty/carto will have its own personality (for lack of a better term): diff vapor, diff TH, diff amounts of flavor, life spans, etc. Ultimately, I think each vapor has really no choice other than to experiment and find which combo of mods, volts, attys and juices are best for THEM.
So, if we assume the 5V GLV2 is 5V (I'd have to do an under load test to verify as the resistors require an under load test to verify but I'll take Jay's word that these are pushing our 5V under load.
Our formula: (V x V)/ohms=watts
Let's assume a regular 510 atty at 2.5 ohms: (5volts x 5volts)/2.5 ohms = 10 watts This is certainly the upper end of the watts (heat range) and can risk that burnt flavor especially if your juices are not shaken and/or have a lot of VG in them. Some people LOVE this level (not me - but some people).
For S&G, lets look at a mid range LR atty (should NOT be used on a 5V device), we'll assume 1.8 ohms:
5v x 5v/1.8=13.8 watts - WAAAY hot and will likely pop the atomizer or burn your juices.
Lets look at a mid range 510 (assume 3 ohms): 5V x 5V/3=8.33 watts (my preferred sweet spot is between 7.5 watts and 9 watts so this is nice)
Lastly, a 3.3 ohms 510: 5v x 5v/3.3=7.57 (also very nice) These numbers would also apply for most 901s and many of the 801 series. My preferred is the 302 and those are usually 3.1 ohms or 3.2 ohms: 5V x 5V/3.1=8.06 watts or 5V x 5V/3.2=7.8 watts.
Hope someone finds this helpful.
BTW, you can use the same formula for any voltage if you just assume the rated voltage is a generalized average (again, these fluctuate with battery charge levels and this is more of an issue with unregulated/unresisted as you usually get pretty close to whatever is being pushed from the battery - still, other fluctuations but for generalized calculations): 6V x 6V/3.3=10.9 watts ----- 6Vx6V/2.5=14.4 watts ----- 3.7V x 3.7V/2.5ohms=5.47 watts ---- 3.7v x 3.7v/1.8ohms (LR atty)=7.6 watts.
Once again, this is NOT intended to be exact! This is just a generalized shortcut to help choose and/or experiment to find what you like best. There are many variables in vaping and it can be difficult to isolate some. You really just have to experiment.
Happy
Might be worth printing out and keeping somewhere for future reference. I'm stuck with all that in my head - and more 
