27.5 x 4.5" is a lotta tire on the front of any bike.
I have ridden and loved this tire in winter, on hardpacked snow and ice. It is so surefooted, so confidence inspiring, that it's hard to imagine a tire I'd want to ride more in those conditions.
That said, those conditions don't come around very often. Now that I have Mastodon on the front of my Fatillac I can't help but to install any and every obese tire to see what it brings to the table.
I've done a handful of rides now with the bike set up as such. Roughly 10psi in each tire -- apropos given that I'm largely riding tech trail and not soft surfaces with it. Could probably drop as low as 8 up front without appreciable self-steer, and I've ridden it down to ~2.5 on a wider rim, on snow.
I don't know the exact difference in weight between the Gnarwhal and the B Fat (~3.5") Hodag that was on there previously, but I'd guess close to 8oz. Numbers aside I can immediately tell that an increase has happened: the bike just wants to go straight, and the faster you're going the more this is so.
No real surprise there -- gyroscopic inertia and all that.
And the traction? Sheeeee-it. It's stupid, in a word. Blow a line and it doesn't notice. Deliberately leave the trail to ride chunky wash, wallride a slab, descend a pourover -- it just eats it up and farts out a creative blend of silt, sandstone dust, and bentonite crumbs as if to say "more please".
Again -- no real surprise there, given the massive air volume and massive square edged blocks.
All that said, the word that comes to mind over and over as I ride this tire is "overkill". It's fun to have a tire creating incessant easy-button situations for you, but at some point you realize that the added mass and lack of need for input are removing some of the challenge of riding. And I know that challenge isn't IMBA-approved these days, but that doesn't mean I don't crave it anyway.
Conclusion: Awesome tire for snow and ice. Awesome tire for off-piste 3-season riding. Massive overkill for a rider with any level of skill beyond "IMBA Expert", which is somewhat similar to "Owned my first bike for 2 whole months now...".
Don't hesitate with questions.