seriouspost: working on INT saves is pretty huge as well -- monsters tend to have garbage INT. I'm running a debuff/control Bard and I run into low Int a lot more than I run into low Wis, or even low Cha.

As a control spell, it's also notable in that it doesn't allow saves every turn: the monster needs to burn its action to check out the illusion, which requires it knowing it might be dealing with an illusion. Since it likely has low Int, its odds of figuring it out (and passing the save) are pretty low. If Phantasmal Force works, it's likely to work for the whole minute.

It's only a 2nd-level slot, it targets a stat that's generally pretty low, it tends to last a pretty long time (relative to combat speed) and it has a wide variety of uses depending on how creative you get with it.

Generally, that amounts to "prevent monster x from doing y for 60 seconds". Medusa? There's a swirling mist of shattered mirror glass around her head, that coalesces into a mirror every time she tries to use her stare. Demon? A sigil of whichever god it hates the most appears, hanging in the air between it and you. Nature-loving druid? A sad puppy whines out in pain from somewhere nearby and in dog-language it cries out for his help, then runs away from him when he tries to approach.

You're trying to annoy, to distract, to stall. It's not a spell for killing monsters, but it's an excellent spell for keeping them busy and keeping them off your party.

Phantasmal Force, like all illusion spells, is worthless unless you get creative with it. If you get creative, it gets crazy pretty fast.