I have a friend who has one for his M11/9 and have shot it multiple times.
I also have a bunch of other 5.56 caliber machineguns as well, multiple M16s and HKs, AUGs, AC556, FNC.
The Max 11/15 is a really nice to have option for somebody who only owns a M11/9 and an M16 or HK sear is financially out of reach. However, I personally have never had the desire to purchase one for my M11/9 either.
Its not that the Max 11/15 is bad per say. They run well, are reliable, take M16 magazines, etc. Richard did a really great job getting a 5.56 upper out to M11/9 owners. However, the Max 11/15 also doesn't offer anything that an M16 or HK 5.56 machinegun already does better IMHO. While the MAx 11/15s are not super expensive they are not cheap either at $3K.
As you mentioned they won't win any beauty contests and the Max 11/15 suffers from many of the same challenges that make the M11/9 not the worlds best subgun out there. The M11 grip profile and grip angle isn't the must comfortable, trigger pull is pretty marginal, fires from an open bolt so first shot on target accuracy suffers a bit.
However this is coming from somebody who also doesnt really have much desire for one of the many different Lage 9mm uppers either. I leave my M11/9 in fairly stock form and sometime swap in a tungsten bolt with a suppressor. If I want a really smooth shooting subgun I have MP5s or if I want to shoot a slow ROF open bolt subgun I have a bunch of other options to pick from. I appreciate the M11/9 for what it is, being a relatively small high ROF subgun.
Personally if you already have an M16 and HK 5.56 machineguns, I would much rather put the $3K 11/15 dollars toward a $5K Shrike/MCR upper.
If you don't have a belt fed, the Shrike/MCR will give you belt fed option for your collection vs. the 11/15 which just gives you another magazine fed upper that won't be as nice as the 5.56 mag fed guns you already have.
The other option would be to get a different HK host for your sear like one of the 7.62x39 PTR guns. The 16" full size and 12" K guns in my experience is pretty plug and play once you swap the bolt carrier out for a full auto tungsten fill unit. They have a really smooth recoil impulse, slow ROF, and gobble up cheap 7.62x39 steel case ammo so make for a great range toy. They have a really different personality vs. either the 5.56 or 308 guns for not much money.
Good luck with whatever you choose. The 11/15 is a nice option, but with what you already own I would see the 11/15 probably just gathering dust vs. being shot on a regular basis.