Not many items make it straight from the draw to the table, Magnifying Glass does just that. A 1 cost permament boost to while investigating is crazy value. it can let you hit the ideal sweet-spots of difficulty for several game modes and once you start layering more bonuses you'll be able to brute-force clues very quickly.
The fast mechanic is a huge deal here. often you might hesitate to play a Flashlight not knowing if you need it right away and when you finally do play it it might set you back one of the actions you'dd rather have used to actually start investigating, Magnifying Glass is out on the table exactly when you feel like playing it and generating value immediately.
The only drawback is the slot. A wont mind, they typically don't have much competition in the slot and might even supplement the Magnifying Glass with Flashlight to raise the odds of drawing helpful hand slots faster. Non characters however might have some competition, mostly from weapons.
Ultimately it depends on your character and difficulty. A 4 is much more useful in standard then hard, so a Magnifying Glass wielding Roland Banks is a solid plan, but on Hard you'll need a lot more support to warrant the Magnifying Glass card slots.
The grand test for usefulness is how great a number you can hit with just the one Magnifying Glass on the table (Drawing 2 is not impossible, but the hand slot might be occupied and if the card is'nt useful with just 1 copy of it in play then it's probably a bad fit for your deck). Netting 3 isn't worth the trouble, 4 is good on Standard but needs more support for Hard, 5 is definitely powerful in all modes and 6+ is an outright autotake.
When you decide to go with Magnifying Glass in your deck make sure you have more support to supplement it with, Perception, Fieldwork, some icon cards, stuff that might buff your investigates to hit the comfort numbers.