Look, HoMM 3 was impossible to follow up. HoMM 3 took everything that 1 and 2 did well and pretty much perfected it. I'm not saying that this game is perfect, because it ain't--but it makes some fun choices that really expand your options, and I'm sorry that it failed the way it did. I still play it and I still like it.
Rodney666's review is pretty much right dead on. It adds a slew of new content, which is something to be excited about--but it definitely does change the flavor of the gaming experience. Nations don't colonize in neat, contained lines anymore; they'll push hard towards your base, and you can expect scouting and raiding parties to start in earnest even on the easier difficulties. It's hard for me to complain about, just for the sheer volume of content--but it's definitely designed for a certain type of play, and I don't much enjoy that type of play. I wanted to take my time and turtle, or else start out with a midgame summon right out of the gate... *sigh*. Oh well.