Ok, I bought the Gamecube version when it first came out. I bought a 2nd copy to replace the first which was destroyed in Hurricane Katrina. I still have that copy. I bought my friend the PS2 version a year ago and he loved it. Now he's my roomate (he still has the PS2 RE4 but no more PS2...sigh) so I'll eventually get to play all the extra stuff the PS2 version had..... until....
I bought the Wii version yesterday and BAM, I get the best of both world, plus the added "light gun" controls. Now I gotta say.... if you have the Gamecube version already, the Wii version is worth it for the added content (from PS2 version), the new better shooting controls, and the $30 pricetag. Now if you have the PS2 version, then it may not really be worth it (unless you REALLY care for slightly improved graphics and a Wii control scheme).
I figure that I'll eventually give my gamecube away to a friend, and I can give them RE4 with it. My roomate will have his PS2 copy, and I will have my Wii copy. I don't regret buying this game this many times for this many platforms (I actually bought more copies for other friends as gifts, the Wii version makes it my 7th RE4 purchase, and I may buy someone else the Wii version).