MGS 5 is fun as a stealth game for 10-20 hours. The story is bonkers, and after mission 30 (?) or so you are just repeating missions with minor tweaks - like have to redo mission 18, but this time stealth only, etc. The game is clearly unfinished, although it "ends" its not very satisfactory. I actually stopped playing at mission 45 and just traded the game in (game goes up to mission 50 or 51), just got too frustrating and repetitive for me.
I still really, really enjoyed missions 1 - 23, put in a good 30 hours, and love the environments, base building, character interactions, and character abilities. Never played a MGS before except maybe one as a kid (don't remember it really), I'd say for $20 you'd definitely get some fun out of it.