I owned - traded for - borrowed - used etc something around 100 different mods. My alter ego on another forum wrote reviews for almost 2 years.... After my first REO, I only bought 3 more mods. Two of those were REOs.
That said: I still own a couple of rough stacks. I don't use them. I'm just emotionally attached to them because that's how I started vaping and stopped smoking. I own a Kalamazoo Triple-V that does once in a while get fired up for a little dripping session. I own a mostly broken VVPV... If that thing worked like it once did, I'd use it. It has problems and I still even after 2 1/2 years have a little anger about those problems... I'd love to type a rant, but it just isn't appropriate. I sold, traded away, and gave away the rest. I don't even have an ego battery laying around here any more.
After the VVPV, I bought my VV Woodville. I didn't even experience desire for any other PV for more than 2 years after that. None. Not one single mod interested me even for a second. ... Until Rob said "Skeletonized REO Grand". He said that and I started saving every extra buck I came across. He had SL Grands on the site for maybe three months before I had enough in my paypal account and every day of it was spent anticipating the day I'd have the spare bucks squirreled away in my paypal to get one. A few weeks ago, I did.
So... My roundabout answer is: I almost exclusively use REOs. Once in a while, I drip a 3.0 ohm commercial atty for an hour before bed on my 5.0v Triple-V. My primary PV is a REO (SL Grand newest). My backup PV is a REO (REO VV Woodville - 2 + years old and was primary until SL Grand arrived... going in the mail to see Rob for "spa treatment" tomorrow) . And my other backup is a 14500 Mini (owned since Nov 2011... backup is a strange word since I'm vaping it right now and it has been used a whole lot ever since I bought it).
I can't actually see any evidence of *needing* a backup if you have a REO. That 14500 mini I just mentioned has been in service for 33 months and has never once failed to operate. It has never once given me the slightest bit of trouble. It has worked every single time I pushed the button for almost 3 years now. ... If you have a "back up", I think it should be just as durable and reliable as the device it's "backing up". That means another REO.