Ye ive played Smash Bros (actually played it fully twice as i first got the Japanese version, only to find out a few days after that the American version came out and game saves arn't compatible across regions), unlocked all the chararcters, played through the campaign (which is the best way to unlock every character except for 3) etc. The campaign itself will last you about 7 hours of play in which afterwards you will probably not want to play it again as it gets pretty repetitive (though the cutscenes are frigging awesome)
It has classic mode, all star mode as well as all the other good thing melee had like target tests, events, home run contest and multiman brawl.
The lasting factor is the multiplayer really, I haven't had experience with online play (as my connection at home sucks due to firewalls and lots of other complicated issues), but me and my friends have been playing it for about a month and it hasn't got old yet. If you liked all the tricks you could do in melee (ie wave dashing etc), you may be dissapointed as a lot of them were taken out of brawl, however if you like playing with friends/online people, its as much fun as melee really.
Hmm, I'm unsure on what import games you would want tbh, as unfortunately the wii doesn't have a large array of "good" games (they'r mostly gimmicky). Hmm, well there is fire emblem and bleach:shattered blade which arn't in the UK, but aside from those I'm generally unsure. However , remember, that because we have a much more awesome currency than the US, their games will be usually be cheaper than ours