Interesting, I’d not heard of them at all.
I’d be interested to hear what other folks think of these kits too.
They appear to be based on the workbee, the bit at the bottom of the page where they thank Carbid3D and the Shapeoko as being an evolutionary ancestor of the workbee and their queenbee might ruffle a few feathers here 
Looking at their extrusions I think they’re a step down from the Carbide aluminium girders. When I estimated the second moment of inertia for the Carbide extrusions I got about 106x10^4 vertically and 58x10^4 horizontally (mm^4 is the unit). I don’t see any engineering spec for the extrusions so I went Googling and found this data sheet from Misumi
I consider Misumi a good source as I’ve seen quite a few custom CNC machine builds using Misumi rails and the builders have mostly said they were very flat, straight, well finished, square cut etc. Also Misumi has a strong reputation in general.
Real mechanical engineers, please butt in and tell me I’m doing this all wrong here…
In the data sheet, pg 2-521 I see the Misumi 4080 in 40mm x 80mm with no C beam cutout and that has second moments of inertia of 18x10^4 and 63*10^4 respectively. So that’s a downgrade of 58 down to 18 and 106 down to 63, given that you’re planning to run a longer beam, not so good. The QueenBee is also C beam so even weaker with some horrid twisting modes too and we know how the Shapeoko Z axis loves to apply a twisting load to the X beam.
Looking at the Misumi 6090 on the other hand that claims 52x10^4 and 111*10^4 which is more like it.
Most of the stiffness of these beams comes from the outer surface material, that’s why the Carbide extrusion is so strong, the 5mm wall on the outside. If I remember correctly from the calcs, if you made it from solid aluminium it would only be about twice the stiffness. All the twiddly bits inside the 4080 don’t do much to add to stiffness.
The statements about bolting extra bits of extrusion onto the main beams adding to stiffness I don’t buy. They are not sufficiently rigidly attached to form a single member in bending, they may well absorb vibration energy through friction at the contact surfaces though. (Again real mech eng please do correct this if it’s wrong)
Otherwise on the QueenBee machine, I think the linear rails are good, but then, I would say that, I have a slightly modified version of Dan Story’s rails upgrade
See Dan’s thread about the rails here
My personal view is the X axis rail upgrade is most of the benefit with the Y being a secondary if you want to go the whole hog.
The leadscrews in the QueenBee design, well, let’s just say I’d go kevlar cored GT2 belt or ballscrew.
Hope that helps.