After sitting out for around 15 minutes, this bag has an impressive amount of loft.
Rab gives the Neutrino 800 a lower limit of -4F. While local temps didn't dip below zero during our testing period, we suspect we'd be pretty uncomfortable at that temperature, and would probably want an additional down puffy jacket and booties around 0. This bag feels warmer than the 650 fill Thermarest Questar 0, the Kelty Cosmic 0, and the Nemo Sonic, all bags with around the same temp ratings as the Neutrino. The Western Mountaineering Kodiak MF and the Feathered Friends Snow Bunting have ratings close to the Neutrino but feel warmer.
This bag isn't as warm as the similarly rated Western Mountaineering Kodiak, but it is less expensive than our Editor's Choice Award winner.
The Rab Neutrino 800 weighs 45.65 oz or 2.85 lbs according to our scales, which happen to be the exact same weight as our Editors' Choice Award winner, the Western Mountaineering Kodiak MF. However, the Kodiak is warmer, uses higher quality down, and has a roomier cut which makes it more comfortable. Because the Kodiak uses lighter materials and better down, it has a better warmth-to-weight ratio. The Neutrino has a good warmth-to-weight ratio just not as good as the top performing bags. It's worth noting that the Neutrino costs significantly less than most of these bags.
This bag weighs 45.6 oz after you subtract the weight of our stuff sack.
The Neutrino 800 feels much wider than its measurements suggests because it's less tapered than some of the other bags with similar measurements like the Western Mountaineering Versalite and the Feathered Friends Snow Bunting. The slight taper makes for plenty of room to roll around if you're a smaller person. Extra room in a sleeping bag will let you sleep in a variety of positions, but remember that extra room equates to uninsulated dead space, so when it's really cold you should fill that space with extra clothes and puffy jackets. The extra room makes this bag a less efficient insulator and not the top choice for ounce-counters who are trying to move fast in the backcountry. It does make this bag a better choice for base camping and long trips where you'll be spending a lot of time in your bag.
The elastic drawstrings are very stretchy, leaving lots of slack dangling inside the bag when you cinch the hood down tight.
This bag is very compressible for its warmth. It comes with what's described as a "dry bag compression stuff sack," but the sack doesn't have any actual compression straps. We packed it into the same compression sack that we used with the warmer Kodiak and the Big Agnes Crosho -20, and the Neutrino packs away slightly smaller than these bags. If space is a premium for you, the Western Mountaineering Antelope has similar warmth but is more compressible.
For it's warmth, this is an exceptionally compressable sleeping bag.
This bag features down that has a Nikwax hydrophobic treatment to help the down maintain some of its loft if it gets weight and speeds up the bag's drying time. We aren't ripping open these $500+ sleeping bags and looking at the down, so we're not sure how much weather resistance we can really attribute to the Nikwax treatment. What we can see is how well water beads up on the shell fabric and how long it takes to wet out, and the Pertex shell fabric does a fine job of repelling light mist and melting frost, but we don't recommend sleeping in this bag (or any bag except maybe the Marmot Col -20) in a heavy rainstorm.
A Pertex Quantum shell is our first defense against moisture, with Nikwax Hydrophobic down for back up.
We like the Neutrino's small zippered internal pocket that will keep your phone, headlamp, and a few camera batteries warm, dry, and easily accessible while you sleep. We don't like this bag's hood and collar cinch cords. They are elastic and are very long, so when we tightened the cords to snug the hood around our faces, it left long loops of slack that hang uncomfortably in the bag. We prefer the hood of the Western Mountaineering Kodiak MF. The cinch cords down the stretch and the hood fit much better; granted, the extra slack issue can probably be solved with some careful aftermarket modification. The zipper snags a lot more than the zippers found on the Western Mountaineering bags.
A draft collar keeps out the drafts, with one cinch cord dedicated for the collar, and another for the hood.
As we suggested previously, the Neutrino is a pretty solid all-rounder, suited for someone who values comfort over weight savings, though the warmth-to-weight ratio is by no means dismal, and we wouldn't hesitate to take this bag on winter backpacking trips.
For the money, $500 to be exact, this bag is a good deal. It's over 150 dollars less than creme de la creme of bags and is pretty versatile because of its low weight and full-length zipper.
The toe box provides ample space for our testers boot liners. He likes it.
This bag isn't quite the screaming deal as its heavier and less expensive cousin, the Rab Accent 900. It does provide a less expensive option, with good performance, just a notch below the top performers in this review. If you want to save a hundred bucks or more and you'll be facing temps in the teens on your adventures this winter, you can expect to sleep comfortably in the Neutrino 800. It's a great bag, but there are a few higher-end models that outperform it.