I love my flex and will tell you from my previous experience with a PC, this is hands down the best DA I've owned. I also have the Griots 3" ROP and while it is great for it's intended purpose, the flex is phenomenal. I'd recommend taking a look at gloss-it.net and look at their 5.75 flat pads.
As for the flex use, I would +1 what some others have already said. You will still want to do a small section (2' x 2') at a time, maybe a bit bigger because the flex will correct quicker then a PC, but slower then a rotary, except with less chance of burning paint. Look for Mike's article on when paint gets too hot. I can't remember the name of the thread right now, but it talks about our natural instinct to remove our hand if something is too hot. He uses this as an example of when the paint is too hot underneath where you are buffing.
As for other tips and tricks. The flex is pretty straight forward. Work in overlapping fashion, across and down, then down and across. Hopefully that makes sense
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Repeat.