That was going to be my follow-up question, whether or not you guys felt them necessary and/or how difficult is it to transfer 5-gallons without one.
I do believe i have the know-how to install one myself, but may just go for one with a valve already installed just to not have to deal with it.
 transferring 5 or 6 gallons without a valve is certainly do-able, just a pain.  picking up and pouring a pot with 5 gallons of liquid is a solid upper body workout.  use an extra-large funnel (available at homebrew shops) to get your cooled wort into a carboy, if you use carboys.  obviously less of a concern if you use buckets.  those funnels are also useful for filtering out crap and they aerate at the same time.  
i put a valve on my pot a few months ago and have been loving it.  it's not a necessity but sure makes life easier.  instead of needing to lift out my bag and let it drip, i can just open the valve and let the the wort drain out.
good to hear.  I've been reading up on it and figured I'd give it a go, but never actually known anyone who does it.  Honestly didn't know it was a viable option until the Zymurgy article last month which dispelled a lot of the common misconceptions I'd been hearing.
An 80% efficiency is pretty awesome.  That's without sparging?  Just letting the bag drip?
 no, i batch sparge.  pretty sure i wouldn't get that efficiency without it.  i mash my grains with 1.25-1.5 qt/lb water, then use whatever water is left for the sparging.  so i might use 4 gallons for the mash, and then have another 4 gallons or so left for a sparge.  
if you ever wanna hang out during a brew day at my place, lemme know.