I agree that Kadees are very effective and have added them to all my North American models, mostly No.5s but the Kadee range caters to all sorts of anomalies as well.  But I have decided to take them off my British stock and go with three link/screw couplers as they really do look genuine even if they are a fiddle to work with.  Some stock (from any of the major brands) can be a real pain to convert to 3 link, and most RTR locos don't have the space for the springing behind the buffer bar.

The NEM pocket concept actually makes Kadees easier to install this side of the pond.  All you have to do is work out the length of the unit versus your track radii.  I have no idea how to do this!

Having spent 20 odd years fitting Kadees to British stock may I make one somewhat pedantic suggestion. Now please bear in mind that I am VERY, VERY fussy and I want my Kadees to work 100% of the time - NOT 99%!!

In that context I would recommend keeping to the NEM fixing style of couplings and NOT mixing them with the traditional Kadees made for the American market. Of course they will work together but will not uncouple 100%.

This means only having stock with NEM boxes or grouping stock into rakes with 18 or 19 Kadees on the ends.

Of course people do mix the styles and are very happy with them but in extreme situations they do behave slightly differently so you will be better either using the NEM fitting or the traditional fitting without mixing.