As a long-time user of a Filco Majestouch 2 (MX Reds) bought from Keyboard Co something over 7 years ago, this keyboard was quite the step change.
My first buckling-spring unit, cannot compare it to an original IBM version as I haven't experienced it.
What I can say about this board, is that it is just bliss to type on. Much more of a satisfying sound than the shrill, sharp click of the MX Blue switches and their clones (especially some of the cheaper ones like Outemu!) - albeit still very loud, but I'm sure if you are after a clicky keyboard you know what you are getting yourself into in doing that.
Keyfeel is very consistent across the board, stabilisers are on the better end of things despite being so simple on the non-space bar keys, somewhat heavy. Caused me to miss some presses at first, but soon adjusted - coming from very light linear switches this is no surprise. The tactility is very definite but not overpowering.
Rollover is 2 keys on here, but you will find combinations that can go higher - can find combinations of 8 or 9 including modifiers. Generally good around the WASD area, not found anything that caused me to miss any presses yet. Slight oddity in that the right Super key and Alt Graph are switched around, even more odd when it isn't on the new tenkeyless model just launched.
There is one thing to be wary of buying a Unicomp board or accessory keycaps and that is the printing. The quality of the print itself now is excellent, evidently they put some money into their dye-sub kit in recent times. However alignment isn't perfect by any means and they seem to have a fairly low standard as to what will get shipped out from the factory - leading to things like the left Super (Windows) key on my board having the legend printed practically against the bottom edge of the cap. However, Keyboard Co sorted me out on that one, so no complaints there :)
The keys being dye-sub will need some substantial action on a sanding disc to wear off those legends - not like some of the cheap pad prints that are out there. And being PBT caps aren't exactly prone to wear either! Unicomp have switched to a single-piece cap on the new production boards, rather than the two-part arrangement that they used to use, and this does result in a mildly different keyfeel and sound. I'm very happy with the result of that though.
The housing is generally fairly sturdy. There is still a metal plate underneath the membrane in these, although I understand it is of a reduced thickness compared to a real original 80s M. Some flex in the actual housing too, especially if you push down below the F keys. This means there is a little deck flex but it is insignificant - you basically have to grab and twist the board, or push fairly hard in the aforementioned area. I really cannot notice any whilst typing, so believe it is plenty sturdy enough. Still weighs a fair amount too!
The footprint may have been reduced from the original M, but this has been done by trimming down the bezel edges, especially towards the rear. The key area I believe is still all the same - so it's still a bit of a chunker! Really quite surprising how big it is, but that's part of the charm.
The only real bugbear I have with this unit is the intensity of the lock lights - absurdly bright blue LEDs. Generally not a huge problem sat behind the board in the normal position but lean forward slightly and you'll get a bit of an eyeful. Odd design choice, as is the woefully ugly legendlogo plate around this. However, you can get swappable ones of those, including blanking it off altogether if you so wish. I think it stands out less on this model in the black, so fairly tolerable in my opinion.
All in all, this is an absolutely wonderful keyboard, and would highly recommend it to anyone after a tactile experience that is just that bit better than you can get with even the clicky type MX-style switches.