The handle is huge relative to the blade and for many that is where the love comes from. One advantage is you get the kind of leverage you would expect with a 4 inch folder, but in a blade size that is legal in more areas and generally more PC (the Millie will make someone uncomfortable real fast).
While the knife is not compact, it's still thin enough to carry well. You get significantly more control with the PM2 than you do most folders with its blade size. That extra control and grip is very useful for harder work. Additionally, it is more comfortable for extended work (I would argue a lot more comfortable) and it can help reduce the fatigue that some smaller folders can have.
While the blade is smaller, it's great. The FFG is great performing and easy to maintain. Between that flat grind, S30V, and the blade shape keeping a lot of steel on the edge, it's tough and the edge lasts a long time. The tip is capable of very detailed work requiring a fine tip, but the blade shape and grind gives the tip more toughness than many other folders. I also like the height of the blade, especially for certain types of food prep and 'rocking' cuts.
The lock is also great. Like AXIS, there are a billion different ways to open the knife and it helps make the knife easier to use in awkward positions (such as being in a tree and having to grab the knife from your right pocket with your left hand with a 'Spyder-Pinch" and do a wrist drop while pinching the blade. The lock also does a great job accounting for wear over time, is super easy to clean, and super easy to take apart and put back together; not to mention, it is a very, very strong lock.
Like the Griptilian, the availability of aftermarket parts, and the extensive amount of upgraded steel models is a nice addition as well. Also like the Grip, the PM2 is a very reasonable price for what one gets for their money.
For someone who wants ultra compact, it's probably not their cup of tea. But for someone who wants a hard utility knife capable of larger tasks, it is a good option. It's worth trying if you use a knife in a fashion in which you think the additional leverage may be useful and warrant the larger form factor.