My first GBA was the Nintendo Gamecube's Game Boy Player. I remember the only way I could play Pokemon Ruby was on the TV with the Game Boy Player, lol good times. I was 8 at the time.
Then in Christmas 2004 I finally got an actual GBA, though it was an SP. It was blue, and I liked it. I was happy that I could finally play Pokemon Ruby on a handheld, and thinking it was pretty cool that I could continue on from my save file from playing on the Gamecube. The next GBA game I got was the Sonic Advance + Pinball Party combo pack, it was either that or Pokemon LeafGreen. Then sometime later I got Pokemon Emerald (obviously I'm a Pokemon fan XD), and I got Sonic Advance 2 and Sonic Battle(this game sucks) from a friend.
I recently started using it again because I realized I never truly got to experience the GBA's library, and I can't stand playing GBA games on my DS lite with AceKard + EZFlash 3in1, because the border around the screen when playing GBA games annoys the crap out of me. So I physically modified my 3in1 cart to fit inside of my GBA and be able to take it out easily, and I flashed the Mother 1+2 translation to NOR, so I've been playing Mother 1 on my GBA SP. Though I just got an EZFlash IV, so I'm going to re-modify my 3in1 back to it's original size, and I'll put all my GBA games on there. I also just ordered a new GBA SP, but the AGS-101 model with the backlit screen.