You have to purchase the games from the shop, but the coins are so easy to get its no time at all before you have them all. Then as you play through each game, challenges for that game become available at the shop. There are 10 challenges (2 sets of 5) available for each game, and they are all heaps of fun. In some ways, they are better than the games themselves (as they are really short, can be downright difficult, and highly addictive!).
Of the games themselves, my favourites are Omega Force (Scrambler), MilkRun (a 2D platfomer), Treasure Hunter (Pitfall) and GalHero (a cool space-shooter ala Galaga).
Merchant Galactic also stands out - its the most unique of the games for sure. Its like a simplistic version of 'Elite' - where you travel between planets, buying and selling cargo. Not really an action game, but Im glad it was added.
Like the promo says, the games are NOT clones either. You can really see where they got the inspiration from, and how they extended the games further.
I love the stat tracking - if this was another 'lame' GBA title without battery save, that would have killed this game right there. But it has full saving, and records your progress through each game individually, all the challenges and the game overall. Gonna take a LONG time to reach that 100% completion :)
Some of the presentation and graphics could be better - but it never affects the gameplay itself. Also I think the music rocks, there is even a hidden music jukebox in the game (shows how many music tracks were made for the game!).