I've got them on my -6, they provide an additional 4.4-4.5 gallons per side. Gravity/air pressure feed, they work great.
Are they needed? 90% of the time, no. But, I did fly from central Florida to Charlottesville, VA non-stop with 16 gallons aboard on arrival. Considering winds, weather and IFR alternate/reserve rules, they provide a really nice reserve. Not sure my comfort level would have allowed me to complete that flight non-stop with only 7 gallons left without the ER tanks.
If I were building another RV, I would buy them again.