Even if you played the heck out of the Gamecube version, it's worth replaying RE4 on the PS2.
Gameplay is just as great as the Gamecube, but with much more extras. The new Ada mission adds a good 5 hours and there are a few more weapons for both Leon and Ada to use.
Graphics aren't as sharp as the Gamecube and the PS2 cuts corners on the secret costumes during cut scenes.
Sound is great. So much tension and great sound effects deliver a terrifying experience.
Value is awesome. All the added content at a lower price than the Gamecube version make Resident Evil 4 worth owning. There is a lot of replay value to try out new weapons, costumes, and difficulty.
Overall, even if you played the heck out of Resident Evil 4 on the Gamecube, it is worth playing the game over on the PS2. However, if you're one of the few who never played Resident Evil 4, you should go buy this game right away.