When this game first came out in the early 2000s, I think bought the game thinking that it was a straight up sequel (or a side story) to first Lunar game, Lunar: The Silver Star, but Lunar Legend for the Game Boy Advance is actually a remake of Lunar: The Silver Star, although I believe some of the events and story elements from the first game have changed a bit.
I missed out on the Lunar games when they originally came out for the Sega CD. I was never able to get my parents to buy me a Sega CD for my Genesis console back then, but I did want one, for games like the Lunar games and other games like Sonic CD, Popful Mail, and Vay.
So I was glad that the Lunar games came out for the original Playstation. I bought both Lunar: Silver Star Story Complete and Lunar: Eternal Blue Complete. I enjoyed playing both titles and I loved extra stuff that came packaged with each game.
Despite having already played Lunar: Silver Star Complete, I also had a very enjoyable time playing through Lunar Legend on my GBA. If you want to play Lunar on a handheld system, then Lunar Legend might worth picking up. However, if you own a PSP, you might to look into picking up Lunar: Silver Star Harmony, which is also a remake of the first game.