Original:
Pro's - quality, fit and finish, warranty.
Con's - Price and sometimes availability.
$20 "silver box" clone:
Pro's: quality, fit and finish, most parts for the original work with this clone. Spare o-rings.
Con's: Has a yellowish coating over the stainless. o-rings in the top cap fall out easily, top cap over twists past the o-rings slightly.
$15 FastTech clone.
Pro's: 2mm airflow base. Drip tip adapter, steel drip tip. Adjustable 510. O-rings don't fall out of the top cap. Threads are really nice.
Con's: The finish is terrible. It's unpolished, so all the edges are sharp and unfinished. Red o-rings. Missing juice control adapter, missing o-ring under top cap.
It's only great for spare parts.
Edit: Of the three attys listed, I can only compare them to the Kayfun.
Lighter, easier to position the coil, double the juice capacity in a smaller footprint. Toolless refilling. When wicked correctly, leaks less. Airflow is way easier to adjust on the fly.
The negatives. Proprietary driptip, somewhat finicky with the wicking. Looks "cheap" compared to a Kayfun. Short 510. (Typically nonadjustable 510, though many of the clones have an adjustable 510).