They are a multi-tool fanatics fully functional play toy. I have a Byrdrench, and it is VERY functional if a bit cumbersome to use. The blade is very sharp, easy to get to, locks up tight, and is a good size for cutting tasks. The slip joint pliers, while small, have come in handy many a time (also, for those that wish this tool had a bottle opener, the pliers are the PERFECT size to grip and pry off the cap

). The crescent wrench (the most novel tool on the rig) is great for smaller nuts. Also, being able to break down the tool to have a screwdriver to turn while you secure the nut on the other end of the bold with the wrench is pretty handy at times.
That's about it, thought. Getting to the screw drivers is a multi-process ordeal. You have to remove the file and spring, dig out the driver bit, probably replace the file and spring so you don't lose them, do your 4 seconds worth of tightening/loosening, then spend another 45 seconds replacing the bit. If you mainly need a small pair of regular slip joint pliers, a small wrench, and a serviceable blade, then the Rench is a nice set up. Most of us need a little more function than that.
However, I will never part with my Byrd. First of all, it was a gift from my wife. Second of all, it reminds me of my first multi-tool (well, before I started carrying Gerber/LM in the late 90's). My dad bought me one of those cheap crescent wrench tools with the knife blade, drivers, and scissors for Christmas. i remember it well. It was the first time I had a gift under the tree that wasn't from "Santa". It was small, yet heavy. Obviously not a toy or clothes. I was 6 years old and LOVED it...for all of a couple of weeks before i lost it. I hope whoever found it was named Jacob as it has my name etched on the side. 22 years ago, and it is still my most memorable present.
In short, unless you have a very specific need for such a tool or have a sentimental connection to crescent wrenches, the Renches are just not that practical. I carry mine from time to time mostly for the knife. It's also nice that you can break it down like a 1911 and clean all the fuzz out, oil it, and put it back together. Great tool, pretty small niche.