This play is most famous not for the character of King Henry IV, but for the interaction between Prince Hal and Falstaff. That's justly so, because you do have a tendency to forget that Henry IV is in it and he is the weakest part of the play. The most obvious interpretation of the play is that Prince Hal is torn between two ideals of manliness. The first is represented by the hedonistic but practical philosophy of Falstaff, and the second by the honorable but impractical and aggressive Hotspur. Between these two ideals Price Hal eventually finds an Aristotelian golden mean. I think that interpretation is entirely the right one.
The plays language is excellent and the characters appealing and broadly drawn. Its the most cinematic of Shakespeare's plays with a lot of the characterization played out as physical action and a lot of good one liners. I suppose it anticipates Hollywood more than any other of Shakespeare's plays. Its good and clever.
It also contains this quote, which of course I had heard before, but somehow greatly entertained me and shows that Shakespeare is really at heart a modern writer of fiction and perhaps the first to be really so:
I can call spirits from the vasty deep.
Why, so can I, or so can any man;
But will they come when you do call for them?