I've never tried the combination myself, most of my knowledge is second-hand from research papers and a friend at college. From what I've heard, the HTPB does cure, but you have to use a high-temperature curative to get it to work. I don't think it has to be IPDI, but if you use something else, you'll have to adjust the ratio. I think that medium-temp microcrystalline wax works the best (you want it to melt at just under 80c). From what I've heard, it works best to mix the HTPB/curative separately and add it to the hot melted wax while mixing. I think it is at least somewhat miscible, but the temperature is rather important to control.
As for how heavily favored it is, I'd say right now it is pretty obscure but picking up steam. It's an easy way to reduce the problem of wax grain slumping if you intent to relight the motor and allow the fuel grain to maintain integrity during hot days. These are more practical problems that occur when you actually want to use the motors than something most researchers have to deal with, so I think this sounds promising for us amateurs.
Also, many of the experiments that use "paraffin" are actually using a highly refined type of microcrystalline wax that has greater mechanical properties and higher melting points than the ordinary stuff. Centrifugally cast Sasol 0907 is one of the favored types. Standard paraffin waxes melt at between 50-70c. Sasol 0907 melts at between 90-100c and is mechanically rather harder. It also usually works best if cast under pressure or centrifugally, otherwise the high thermal contraction of wax as it cools can cause internal cracking and other troublesome issues that compromise the grain.
Other interesting options involve the addition of aluminium to HTPB or wax in order to reduce the optimum O/F ratio. I've generally not found major ISP gains from doing so, but reducing this ratio can lead to major overall impulse gains as well as also increasing the fuel grain density.