Though DKC is still a great game this port doesn't offer enough new things to make it a good buy if you played the original SNES classic.
The level design. This has been one of the strongest points since this game was released on the SNES. You need to time your jumps carefully and also watch which character you are currently playing because each of them (Diddy and Donkey Kong) has certain weaker and stronger points compared to the other one. You can unlock some extra challenges if you search for all the items as well as new pictures in the new picture gallery. Two new mini games are added including a dancing and fishing game. They are nice but not a reason to buy this if you had the original. Furthermore the sound is also very good even on the GBA. It's still all those brilliant atmospheric tunes from the SNES.
It just doesn't offer enough for the millions of SNES veterans. The new modes are worth a look but you won't find them worth the money to buy this GBA version. It's basically made to give younger players the ability to play this classic. Also the graphics don't look so well. The enemies and heroes are still looking great but the levels (especially backgrounds) are looking horribly blurred at times, and are just ported badly.
The Bottom Line
It's still a brilliant game and you'll love it if you haven't played the original. If you did and are already familiar with the SNES or GBC version it doesn't offer enough to make it a valuable purchase.