A buddy and I just took a couple fat bikes out to see what all the fuss was about and was totally blown away!!
Huh, I just had the totally opposite experience with a 4" fat bike. We had somewhere around 6-8 inches on the trail, and it just wasn't working (semi-packed or totally untouched, didn't really matter). It couldn't dig in and get grip. Maybe it was the type of snow and trail conditions or tires, but 'floating' on the surface wasn't going to get you anywhere uphill or even level. I dropped the pressures from ~8-9psi when I picked it up to quite a bit less (I'd guess 5-6, but no gauge), but no dice. I'd guess on a nicely groomed and compacted trail, they'd be a blast though. On a good surface, that extra width is fun.
I went back out in my ~20psi 2.1" 29er, and bam, much better... sink a few inches to compact the snow, and the rear dug in and would bust the front right through the crust ahead. Still hard work and slow going, but more rideable in depths up to almost the bottom of the rotor. Much above that, I think it's hopeless on whatever bike you're riding. Plow into a 2' deep drift at speed, and you just endo into a pillow of fluffy cold.
I just got back from 12 ish miles at Horse Ridge, about 15 miles east of Bend, Oregon. WHAT A PLACE! Tons of technical climbing and downhill...mid 60s today. So amazing (until I was riding on an old cattle trail and somehow a piece of wire got jammed into my front disc brake and I went over the handle bars. Ugh)
I'm at that time of year when it's still winter on one side of the mountain and spring on the other. I ride up a 3.5 mile service road that's mostly south-facing then HAB up the last mile or so as it wraps around the back of the peak and up to the summit.
<a href="http://www.flickr.com/photos/19516547@N04/8541678059/" title="166 by kentsaundra, on Flickr"><img src="http://farm9.staticflickr.com/8089/8541678059_088ab8e925_b.jpg" width="1024" height="768" alt="166"></a>
Then from here, just over that edge..... it's all steep, downhill, South-facing, mostly dry goodness.
<a href="http://www.flickr.com/photos/19516547@N04/8541670211/" title="168 by kentsaundra, on Flickr"><img src="http://farm9.staticflickr.com/8374/8541670211_7af25211e3_b.jpg" width="1024" height="768" alt="168"></a>
<a href="http://www.flickr.com/photos/19516547@N04/8542767242/" title="169 by kentsaundra, on Flickr"><img src="http://farm9.staticflickr.com/8523/8542767242_663c150c38_b.jpg" width="1024" height="768" alt="169"></a>
We were set to ride to day on bare ground for the first time all winter......it started snowing 3 hours before the ride and we found ourselves in 2 to 3 inches of new snow...absolutely beautiful out in the woods today....here are a couple of pics...Ride On!
There are two paths you can go by but in the long run........