So uh... is there even any reason to use a Invisi Watch over a Cloak&Dagger? I don't much see how it has any advantage unless I'm missing something. And what about the Dead Ringer? I tried using one but I have no idea how it works.
Yes this topic is becoming "YellowYoshi tries every class and fails at all of them".
Dead Ringer is good if you're talented at blending in with your disguises - trigger a feigned death, double back around, and kill whoever 'killed' you.
Invisi-watch over Cloak and Dagger has various advantages - if you're always moving, using your stealth for mobile cover to get behind enemy lines, wreak havoc, and GTFO quickly, use the Invisi-watch. If you're more interested in getting behind enemy lines with it and killing one or two high-value targets (good Medics or Soldiers, for example, or an Engineer maintaining a Sentry nest), the Cloak and Dagger would help. Creep into their base slowly, assume a disguise, kill, and then flee or let yourself be killed.
Cloak and Dagger tactics like that, and Dead Ringer, are best used for one life before swapping classes, because then the other team will be spy-checking and doubting your 'deaths'.
The Invisi-watch, however, is refilled by ammo crates, so if you learn a good path, you can maintain a nigh-endless cloak.
Here, have some
tutorials and examples. Very handy for understanding the uses of an Invisi-watch.