Before you face A, everything is so linear. You just go from town to town one after another. Talk to someone in a cafe about opening up the arena, they open it for you. You dominate and you move on.
Afterwards though, you get to explore the game more by facing people like Nanimon who randomly pops up in different cafes. Then all the Season 1 digidestined and their ultimate level digimon are actually challenging. And they make you actually look through the different cities to further the gameplay.
My only gripe is that if I do get confused about where to go, the walkthroughs on this site are either incomplete or very difficult to understand.
I found it to be the complete opposite. The second half, although more opponents show, bugged me with having to go through multiple Extra Arenas everytime a new partner card evolved. Granted, there's a lot more for you to do and explore, but the 2nd half is essentially you facing the same 6 digimon and their evolve forms 3-4 times each, which is boring as hell. The fun only comes in the collecting of better and stronger cards.
I liked the 1st half's story all the way up to facing A once again (played DW1, was a great surprise to see him again); from Rosemon's mysterious appearances to the Arena's seemingly strengthening you in order to face A (or rather VMyotistmon who's seen as the boss at the time), everything worked together as a whole. 1st half was the game's story mode, 2nd half was just the RPG experience. Sort of apples to oranges in that respect.
Yeah I continued playing and got annoyed at the Extra arenas. Of course it's a whole bittersweet aspect, you do need 300 victories to face Black Wargreymon but 300 wins is a lot and completely unnecessary. Now the only person who poses a challenge to me is Apokarimon who still can't really beat me. So now I need to win almost 100 battles and the game is beyond easy for me at this point.
Still a fun game, just the race to 300 wins is obnoxious.