Blades don't tend to be on the same product life cycles as most normal irons so four years is about normal for the 2018 Apex MB to replace the previous 2014 version.
The new version reverts to a more classic muscle back, which is what the MB stands for, with a full width curved backside to replace the previous more centralised Y shaped muscle.
This puts a little more weight in the heel and toe than before to help the MOI, but let's face it, these irons are not really about forgiveness.
The heads are smaller than the cavity back 2018 X-Forged irons that are launched at the same time.
A closer size comparison is with the cavity back Apex Pro 16.
The Apex Pro 16 is about the same length, but has a wider sole to accomodate its cavity back too.
You can see how this changes the flight of the ball on GC2 by the extra spin that the Apex Pro 16 generates relative to the Apex MB with a 7-iron at 86 mph.
If you need a little more forgiveness and can take a little deeper feel then the similarly sized Callaway X-Forged 2018 irons should give you an extra yard or two and could be blended into an MB set.
Feel is what the Apex MB is about and the 1025 Carbon steel head gives you lots of it with the usual sharp feel that only a narrow blade like this can provide.
The lofts are pretty traditional and it almost seems unusual to see a pitching wedge of 47° these days.
The face features Callaway's 20V grooves which aim to provide consistent spin from all conditions and reduce the effect of fliers for more control and certainly the grip was very good.
The sole has a lovely camber on it and went through even the firmest lies really well and continues the excellent performance in this regard for Callaway forged irons over the years.
Turf interaction is probably a player specific thing as it may depend on your angle of attack, so trying out the Apex MB against the other main forged blades in the market, such as Titleist 718 MB and Mizuno MP-18, to find the one that suits your game best is probably the best advice I can give as they are all the same price.
Therefore it is probably going to come down to how you feel about the performance of each model, its looks and each brand and if you go for the Callaway Apex MB then you won't be disappointed.