Its been getting excellent reviews and the game play and sandbox looks stellar.
If I've never Metal Gear'd before would I be able to pick up and play with this one or am I not going to have a clue what is going on?
You're very likely to not have a clue whatsoever. The game is probably still going to be fun (because the gameplay is excellent), but the story will probably end up being very frustrating indeed. The game assumes, at the very least, a working knowledge of the events of MGS3 and MGS: Peace Walker (which are the games preceding it in the MGS chronology); if you do not have that, you will have absolutely no clue who all the people you're interacting with are or why you should care about them or what it is they're trying to accomplish.
I don't think it's necessary to have played these games (Peace Walker in particular is a very long game in its own right), but I think you do want to read up on what happened or watch the cutscenes on youtube. If you do decide to play them, be warned that you'll definitely need a console, as neither MGS3 nor PW have been released on PC. Personally, I would recommend the MGS HD Collection for PS3 and 360, as you'll be getting all of the relevant titles there (and you'll get the absolute mind**** that is MGS2 thrown in on top).