I just tried Wartech: Senko no Rondo for the first time. Talk about awful. Clunky game controls, and terrible play mechanics. I only tried it because most sites list it as a SHMUP. It's not. It's a poorly made fighting game.
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I couldn't get into Senko no Ronde either, but some people say it's pretty good if you can get past a certain point in the learning curve. There is also a sequel that is supposedly much better, but it is Japan only.
Anyway, I haven't played any Xbox 360 games I would say are especially bad. I've been pretty happy with all of the games I've played. However, if I had to name some of the worst, I'd go with these:
Hexic Record of Agarest War
Hexic is just... well, it just goes on too long. The loss condition is too easy to avoid or delay, and I usually get bored before I lose. In a nutshell, the pacing is awful compared to other puzzle games like Tetris or Lumines
Record of Agarest War had some interesting ideas, but everything was implemented poorly. The dialogue was slow-paced, repetitive, and overly verbose. The exploration areas had the most janky 2D walking I have ever seen, the in-combat sprite art was pretty bad, the battles were too repetitive, and the load times were absolutely dreadful. However, the big multi-attacks and combo system were kinda neat, the multi-generation system was interesting, and I think it could have been a solid game with some editing, polish, and technical improvements.