Taro94: All four premium modules available as extra DLC are much better than the three provided in the base game.
Wyvern Crown of Cormyr is probably my favorite of the bunch. It has absolutely great medieval atmosphere, coupled with some of my favorite music tracks NWN has ever had. It's also the only official module to have horses, and while some hated them, I liked them much, especially the jousting tournament minigame. The ending feels a bit rushed, however. The module takes about 20 hours to finish.
Darkness over Daggerford is probably the longest of the four and is a close second to Wyvern Crown of Cormyr on my list of favorites. It's very different gameplay-wise, however. WCoC allowed you only to travel to areas you need to go to, not letting you revisit old places. DoD is an open world adventure (featuring a mini-map) that lets you travel around freely. It has a great deal of quests and generally is very polished, especially the EE version, which includes voice-overs for major characters and quite a few new music tracks. The module takes about 30 hours to finish.
Pirates of the Sword Coast is still very good, though probably my least favorite. It's packed with humor, however, so if that's your cup of tea, go for it. As the name suggests, it's a pirate-themed adventure and it delivers. I know some people consider PotSC one of the best modules available. It supports multiplayer. The module takes about 10 hours to finish.
And last, but not least, Infinite Dungeons. It's... a mixed bag. It's the most original concept out of these four, being a randomly generated hack and slash dungeon crawler. I personally like it, but it's got abundance of overpowered loot. My suggestion is to use a Token of Mastery (which you need to spawn manually via a cheat you can find in the Infinite Dungeons article on the NWNWiki, unless you beat the game) to customize the dungeon and lower the loot's worth. That issue aside, however, it's a great module if you're looking for a dungeon crawler. It supports characters of any level and also multiplayer, too. Once you have the Token of Mastery, it's also highly customizable (choose level of monsters, loot, number of floors, size of floors...) and has some other nifty features exclusive to it, like the ability to export a dungeon and send it to a friend, or a point system if you like your performance graded. The module itself takes around 40 hours to finish, but it can be played infinitely, as the name suggests.
I'd be happy to play some Infinite Dungeons on multiplayer, if anyone wants to. ;)
All in all, I think all the DLC modules are worth their price, and aside from Darkness over Daggerford, which is more expensive, all of them actually seem like a real bargain, given the length of each of them.
I would not say Infinate Dungeons is original; it's pretty much a updated version of the old D and D SPI classic "Dungeon Hack"/ (also available on GOG.9.