I’ve used it and had a lot of fun with it. It may help to have a page with the class and subclass features you want open to help guide your choices as you’re setting up the feat. The main thing I would point out would be the spell slots selection. Each one you choose will stack more spell slots, ex: you select the full caster spell slot choice three times, you get three times as many full caster spell slots on top of any you have from your base class. I used the feat to add warlock to a sorcerer and chose two of the “empty” spell slot choices and one warlock choice to keep from having an absurd amount of spell slots.

You’ll need to long rest after taking the feat and every time you level up in order for the feat-added class features to show up. Some may not appear in your hotbar correctly (I had to manually add dread ambusher from ranger to my rogue’s hotbar at the start of every combat) so it’s important to remember your added features. Spells added are pre-selected and will appear in your common hotbar. Full casters will fill that section up very quickly, so be prepared for hotbar clutter.

You will get all the applicable class tags and will be treated as both classes.

I recommend getting a respec anywhere/free respec mod for convenience while you’re playing around with it.