It really depends on your situation. If it's cold where you live (below freezing when you fly), then YES, get the update. The risk of over current draw is higher in cold climates, potentially causing the voltage to dip below 3V on one of the cells, which will trigger the battery to SHUT OFF using the old F/W. This is a real liability in the previous f/w revs if you're in cold climates. You'll just have to live with the reduced performance for now, maybe forever with the P3. But if you live in warmer climates, and enjoy fast acceleration, then you may want to wait, assuming you're OK with the drift in 1.4 (I lived with it for months, it wasn't that big of a deal) then stick with 1.4. Or if you're OK with potential vibration issue in 1.5, you might want to stick with that, preferably in warmer climates that are above 45F.
I upgraded because I got a new Ipad
Air2 with IOS9.1 for Christmas. My former iPad Mini2 IOS was 8.4 running an old app and older 1.4 firmware. Since I had to load a new APP onto the new iPad
Air2, I wasn't sure about compatibility with my old 1.4 firmware, so I opted to upgrade everything to the current version. I'm OK with it, I'm just really disappointed. It's like buying a Corvette with a supercharger and having a fun time driving it for 6 months, and then the authorities say sorry bud, no super chargers allowed in this country. What a letdown. You may ask, do I need a supercharger? Maybe not, I'm not sure yet, but I PAID FOR IT! And now it's gone.
Fortunately I didn't see a vibration issue in my craft after the upgrade. That's what I worried about most when deciding to upgrade, but it didn't appear in two batteries of flight. Now I've got a sluggish Phantom 3P, however it's way cool and far ahead of the competition. Yes, I'm still in love with this bird, and my new Ipad
Air2 is the bees knees, pretty nice.
I might go back to 1.5 just to see how I like it. I never tried it before, but I'm in So. Calif, warmer climates, I should be OK.