My experience is pretty spotty, I've only played MGS1, a small bit of MGS2, and most of Peace Walker, but I think it's a series that everyone should give a try.
I don't think MGS1 has fully held up but I think it's pretty playable because it relies on simple controls and straightforward actions, but it has a few obtuse objectives (you'll get stuck at parts with no idea how to proceed). Its pretty ugly and there is loads and loads of radio calls in it that deepen the characters but also drag the game out. Oh and it's basically a top down game which surprised me. I'd definitely say startwith this game from everything I know about the rest of the series. If you don't like it you can probably jump into MGSV without issue.