I bought my 2015 Kona Unit at the end of 2017 from a guy I found on Pinkbike in New Mexico. I thought I might be interested in single-speed mountain biking and was able to pick this up for a good price, so I went for it.
It's fully rigid, rocks 29" wheels, has sliding rear dropouts, and climbs like a goat.
The 2015 Unit runs 29 x 2.25" tires and doesn't have room for much larger in the rear. Recent models have shipped with clearance up to 29 x 3", which sounds dreamy to me.
I haven't pulled the trigger on replacing my frame yet though, mostly because I really like the metallic purple color that I currently have. 2.25" is plenty for now, especially with the kind of mountain biking I've been doing recently (mostly long rides on fire roads). I am considering trying out a 2.4-2.6" tire up front for a bit more float and traction.
I think running tubeless on this bike, or really any rigid mountain bike, is almost mandatory. Maintaining traction is incredibly important when climbing up challenging sections of trail on a single-speed, and running very low PSI helps a ton. I tend to have these tires right around 16-20 PSI for most rides.
One feature of the Unit that I didn't fully appreciate at first is the sliding rear dropouts. Bike chains stretch gradually as they are used, and when you're riding a bike with only one gear and no spring-loaded derailleur mechanism, all of that stretch results in an increasingly slack chain. The Unit solves this problem with a mechanism that holds the rear axle which allows the rear wheel to slide back relative to the frame, tensioning the chain. It's an elegant solution to the problem, and a nice feature of the frame.
I have the bike set up with a 30t chain-ring in the front and an 18t cog in the rear. This set up is a nice compromise that allows me to get some decent pedaling done on flat ground, but I've definitely taken the bike on rides where I would have appreciated lower gearing for the climbs. Especially since moving to California: some of these fire roads are relentless.
I haven't taken this bike out for much bike-packing yet, but I'm sure it would do well. I tend to keep my Tribulus Limited Endover on the bike for carrying tools, clothing, water, and food.
Even with a bunch of stuff in the bag, I don't find that it affects the handling of the Unit too much. It seems to take to a front load fairly well.
Another major benefit to the single-speed is the reduced maintenance. I basically only need to air up the tires, oil the chain, and slide the dropouts back as the chain stretches. The drivetrain is also super quiet compared with my other bikes, since the chain line is always straight.
Single-speed mountain biking is an interesting endeavor that I've learned to love. Without the help of multiple gears, I have to read the trail well, focus on technique, and sometimes just drop the hammer to power through more challenging uphill sections. The bike is a great teacher, and I think it has made me a better, stronger mountain biker.