I had the pleasure of spending a pre-launch weekend in Moab aboard the SB5.5. If I could have brought the bike home with me, I would have, but I'd also need to bring all the rocks home as well. The SB5.5 only further blurs the line of a bikes capability along the trail/all-mountain/enduro spectrum.
Full disclosure, I'm a Yeti guy, the ASR, ASR-5c, SB95c, SB5c, ASRc have all spent time in my garage, and the SB4.5c presently is tenured there.
Forget all you preconceived notions about a "long travel" 29er. "A 160mm fork will feel too raked out and wander up climbs" - wrong. "The wheelbase will be too long and hamper maneuverability" - wrong. "29ers are dumb" - wrong again.
I'm 5'7", and rode a medium SB5.5 with ease and comfort. The Switch Infinity heart of the 5.5 stays true, and will claw its way up anything as long as you keep the pedals turning. Going up the Amasa Back jeep road, and Hymasa trail, the 5.5 felt similar to my 4.5. Efficiency wasn't an issue with the additional inch of rear travel, and the 160mm Fox 36 could be pulled up steeps and step-ups with the ease of my 140mm Fox 34. Where the 5.5 separates itself from its shorter travel sibling, as it should, is once you point it downhill. It'd been 3 years since I'd last descended Porcupine Rim on my then ASR5c, the SB5.5 made 2013 feel like the dark ages. The Fox 36 came to life will all of its 160mm of "keep your teeth in your head" travel. The Infinity link feels supple and bottomless; a coworker took the 5.5 off a 6 ft drop; all he was asking for at the bottom was another bottomless token for the fork. This bikes wants the A lines, and the faster the better.
As Yeti puts it, this is a purpose built bike, and the spec follows suit. No plus-size tire shenanigans on this machine. A meaty 2.5 Minion DHF tire breaks the trail, and aesthetically brings proportion to the Boost 110 Fox 36 fork. The Float X Factory DPS shock sits in back, and will leave you asking "What happened to that rocky section?" A Reverb dropper is stock on all complete offerings, and all come in at less than 29lbs.
Geometrically, the SB5.5 proves form equals function. The medium frame has the same 90mm headtube length as my small 4.5 to keep things balanced, standover is lower too. The wheelbase for a medium is 1168mm, 20mm shorter than a medium SB6, and only 3mm longer than the SC Hightower that uses a 140mm fork. The 66.5 head angle keeps things stable at speed, but the paired 73.6 seat angle makes that fork feel shorter on the climbs.
If you have any other questions about the SB5.5c, or any of Yeti's offerings, don't hesitate to give me a shout. My direct contact line is 801-736-6396 x4378, e-mail is firstname.lastname@example.org