Right, PSPlus in a Nutshell:
- The subscription is necessary for Multiplayer games on the PS4 except for select titles, F2P and applications. It's not necessary for PS3 or PSVita Multiplayer
- In addition to Instant Game Collection, the subscription gives you various discounts on the PSN store. Games which you purchase using those discounts stay on your account forever
- The subscription gives you access to new games monthly, the games stay on your account forever and can be re-downloaded and played for as long as you are a subscriber
- If you choose to let your subscription run out, the games will once again become available to you after you extend it
It's a specifically good deal if you have several Sony systems
or if you plan on getting more systems in the future. Say, if you
"purchase" a PSPlus version of a PSVita or a PS3 game, that game will be available to you if you ever buy either system. Honestly, it's a lot of bang for the buck - I can't recommend the service enough.