I personally think Stopwatch is the most cancerous thing to happen to league in a while. I've always been fine with Zhonya's because you have to dedicate your build to it. But when everyone on the enemy team has one, it can be such a game changing item for a mere 600 gold (or for free if you have the rune). I can't think of any other item in the game that can potentially have such a massive impact, such as turning a critical teamfight (i.e. AD carry getting caught out of position) or causing a dive to go horribly wrong.
I'm on the fence with having it as a shop item since unless you are building toward a Zhonya's, you are setting yourself behind in your build. However, I always thought that runes were meant to be minor/moderate enhancements to your champ, but not something that should make or break the outcome of a game. It's not even a keystone so you aren't really sacrificing much to have it.
I think stopwatch as an item (or at the very lease, the rune) needs to be removed. What scares me is the chance that Riot decides to just give one to everyone. They did that with gold generation several seasons ago... when everyone was buying phil stone and heart of gold (and in some cases avrice blade), they just removed the items and increased gold generation across the board. In a way they did that with wards by giving everyone trinkets.
Granted that a lot of people don't always use it optimally, kinda like how someone burns their flash when they are guaranteed to die anyways. However, it does kinda reward poor play by letting people get caught out of position and not get aptly punished for it as well as not rewarding well-planned plays like ganks/dives simply because someone got a free item for doing but play for 10 minutes.
Maybe I am just overreacting to seeing it being used at games where the highest level of play is on display, but for something that potentially powerful to cost 600/free just doesn't sit well with me.