I am an old school D&D gamer/Dungeon Master who cut his teeth on the 'Red' basic set and then moved into "Advanced Dungeons and Dragons" (1st ed.), all the while trying other stuff like Star Frontiers and I.C.E. By the time 2nd edition came out, I had moved on. Just recently, I decided to get back into it and started up a 1st edition campaign. As the new 5th edition material came out, I bought them initially 'out of curiosity' and have now started a 5th edition campaign. I may move exclusively into 5th edition because of the common sense ideas, ease of play, and stunning packaging. Having the basic set of rule for 5th edition available on line for free makes it easy for new players to prepare before committing to buying anything.
That said, I have to say that I love the "Monster Manual". The artwork is amazing and each monster pretty much gets its own page, with loads of details in an easy-to-read format. In the back of the book is a section of creatures that are not as much monster as wild animal or giant-sized animal and then a section of sample NPCs. In each case, the information presented is easily usable "as is" or in a modified form. I find the information throughout the book easy to reference and access.
From a nostalgic point of view, I still love the 1st editions of Monster Manual, Monster Manual II, and the Fiend Folio for their diverse artwork (some good and some not so good) and background information. The 5th edition is much more consistent in terms of information presented and quality of artwork. First edition had a lot more monsters, but the 5th edition ones are the ones you'd actually use frequently. In essence, this one book serves me just as well as those three volumes did.
Overall, my advice to older gamers who'd think they like to maybe get back into it - start here with the 5th edition. The three core books (PHB, MM, DMG) are superb in presentation and in content. New gamers? In my opinion, the 5th edition is very easy to jump into and have fun with. Start here!