I liked it, but the region itself was pretty forgettable. The most memorable regions, in my opinion, were Johto, Hoenn, Kanto, in that order, with Unova and Sinnoh bringing up the rear. The region is good in concept, but it's missing a certain... something that the other had.
That being said, the Pokémon were good, with Empoleon being one of my all-time favourites, and the concept of the starters having some way to counter the one that's normally effective against it is awesome. With the Platinum 'Dex expansion, the Pokémon selection went from decent, but rather unbalanced, to great. Cyrus was an alright villain. From a gameplay perspective, the physical/special move split was a tremendous occasion for the series, and particularly the metagame.
However, generation 4 introduced one mechanic that's on my personal un-fun list, right up near the top with Eviolite: Stealth Rock. If it were like Spikes, then I wouldn't have a problem. However, Stealth Rock damage is affected by typing and cannot be dodged without something with Magic Guard in the fourth gen, limiting your choices to Clefable or Clefable. Magic Guard was given to more Pokémon in the fifth gen, and Magic Bounce was introduced, but we're not talking gen 5.
Why do I hate Stealth Rock so much? Let's look at it this way: That awesome Charizard that you put all of your effort into raising, because Charizard is your favourite Pokémon ever? Crippled. Stealth Rock takes off 50% of its HP. You want to use a Fire, Ice, Bug or Flying-type? Sorry. All weak to the Stealth Rocks. Even Pokémon that aren't weak to it can still take a significant enough amount of damage. It's just... too controlling of the metagame. It had the competitive scene in a stranglehold. It just wasn't fun. That's why I hate it, and why I will never have Stealth Rock on any of my competitive Pokémon.