Just had the misfortune of encountering my first Tiamat-stacking Fiora. Was midding as Katarina while my bot lane fed her, and by the end of the game I was the only one able to jump into teamfights and finish her. We lost horribly, needless to say. I think a single Tiamat, in itself, is a bit underpowered if anything. That is, the splash radius is surprisingly small to my understanding. Although, I'm not sure if the extra splash is also added to the target you are attacking. That said, the single biggest issue I have with this item is that the splash damage stacks, allowing your attacks to splash into 200% of your AD if you build 4 Tiamats, effectively doubling (or tripling if it applies to your target) your damage output and allowing you to nuke whole teams. I think Tiamat seriously needs its splash damage effect to be a Unique Passive, with a splash radius increase in order to keep it competitive. Thoughts, Ideas, Opinions below.
EDIT: Two Notes to make here.
1. I used the word "Broken", not "Overpowered." My issue with Tiamat is that it is either abusive to game mechanics or otherwise completely unviable, in most cases both. There is not a single instance I can think of wherein it reaches a comfortable middle ground.
2. Please take the time to read through comments before downvoting, I wrote this OP to get the thread started, not to have the final word on the matter. Most of the best posts on this thread are midway through, and are from other posters. The idea is to facilitate discussion, as it is common knowledge that noone buys Tiamat in any competitive scene, and I am curious what others would suggest doing to change this.
RE-EDIT: Looks like Tiamat is going to receive some attention in the itemization changes for season 3 after all. Woot Woot.