Link to the Past was the very first Zelda game I ever played, and I was thrilled to see that it found its way onto the GBA. LTP was one of the better games in the Zleda series (certainly one of the biggest, hardest, and longest anyway), and it's just as good now as it was 10 years ago. Only the sound took a dive due to the tiny GBA speakers, but it's still a great game.
The only thing that drives me nuts about this game is the shear size of it. The dungeons are huge and maddeningly difficult, to the point where you're ready to tear your hair out. Some of the puzzles aren't very intuitive either, and small mistakes early on can leave you stuck and writhing in agony for hours afterwards. This is one of those "so hard I have to beat it so I can brag about it" kind of games. Whether or not that's a good thing is up to you.
I unfortunately have not had the opportunity to play the multi-player game Four Swords due to the fact that none of my close friends even own a GBA. It looks interesting and seems like the same quality one would expect from Zelda. LTP is certainly reason enough to blow 30 bucks on this pak, though. You'll love it for it's great play and plot, hate it for its difficulty, and then love it again when something goes right (which it usually does in the end). Beware that it'll burgle your time too!