The only thing that really sucked about Tactics Advance was it's story. I think what Square-Enix's fourth Dev team thought was that if they were making a game for a handheld for NINTENDO which is primarily geared toward CHILDREN, it might be a little risquè to throw in a long and complicted ( but completely awesome ) story that involves corrupt churches, bloody feuds between brothers, and anything necessarily interesting.
However, everything else was horribly addicting, in my opinion. I found myself dueling clans over and over to boost my clan ranking, and missions were fun and varied to take part in. The interface was sexy for the GBA, and made navigating through the formation an enjoyable affair. Second, I loved FFIX's system of learning abilities from weapons. Sure, it was simplistic compared to FFVIII's junction fiasco ( not knocking it, twas fun, but this is in comparison ). FFTA implements this very well. Law cards and prison seemed to annoy a lot of people, but I was one of the few whom loved it. Sure, who wouldn't like to go all out on another clan? I for one would, but there's no real difficulty in it in Tactics Advance with buffered hit rates and instantaneous spell casting. So, laws and the slammer were thrown in to keep you from annihilating without much thought into it. Some laws are simple to avoid, such as "Addle" or "Wind", and others are a real pain in the arse. "Healing" and "Dmg2: 'Insert Race'" are just two I can think of. Thankfully, if you invest in Ezel's Antilaw cards, you can trump any judge trying to ruin your fun.
Overall, I think that it gets a bad rap because people were expecting it to be better than Tactics. It is by no means better, but not bad by any means. And also, some Final Fantasy fans are spoiled little goobers that if something less than their high expectations comes around, it's dismissed as trash. Trust me, there are a lot worse RPG's than Final Fantasy Tactics Advance and Final Fantasy X-2. Even from Square-Enix. One example: Unlimited Saga. I still have nightmares from it.