I've played and thoroughly enjoyed playing The Walking Dead Season One by Telltale. Therefore, I picked up a copy of the 400 Days add-on. It's pretty much more of TWD with a different cast. A few hours long. I played it. I enjoyed what it offered and I was rather looking forward to carrying over my decisions from Season One and 400 Days into Season Two. However, when I played through Season Two I discovered that the choices I made in 400 Days virtually had no impact on the story. All it did was allow the characters from 400 Days to be seen briefly in Season Two and that's about it. There is one character 400 Days that comes along with you in Season Two but I felt that the character was forgettable and inconsequential to the story as a whole. Pick this up if it's going cheap. It's worth no more than a couple of GBP/USD/etc. As a fan of the Telltale's The Walking Dead, I wouldn't give this a high recommendation.
