I is a charming game that's worth trying just to see the roots of the series. The sequels make spending lots of time with it unnecessary.
You may easily fall in love with II, imho it's the most beautiful looking game ever. It added skills and creature upgrades and also the necromancer castle. However, it has this "tiny" flaw - the factions are ridiculously imbalanced so for example when taking two players with more or less equal skill, choosing the knight against the warlock is like bringing a knife to a gunfight.
III - it all has been said. This is the game that stole more time in my life than any other.
IV - there are some good ideas here and some people tend to like it. Imho it's completely skipable.
V - with Tribes of the East it's a surprisingly good game with a very, very good addition that it initiative (bar). Unfortunately it also has some serious flaws - very few and very small maps, magic that suffered a serious retardation and went back to II level, absolutely hideous visuals to name a few.
VI - yeah...
Now, a quick guide to enjoy HoM&M:
1. Get a version of III that has both AB and SoD in it and latest patches as well (TPB has a version that has it all and runs from a flash drive).
2. Use the HD patch (optional).
3. Get some pals to play with.
4. Play on impossible level. Do not get used to other difficulties, start with it and learn. The game becomes much more interesting then, wars for resources have a huge meaning and there are actual c&c in developing your base of operations (like choosing whether to invest in the town hall or rather get some troops). On hard, normal and easy every game looks the same and is way too easy.