Any tips to playing Widowmaker?
Create a custom game and run around the maps seeing what you can grapple and where you can get to. Learning the maps is huge for Widow. You can also cancel mid-grapple by jumping. And if you jump right at the moment of landing, you can kinda propel yourself onwards. All useful if you're being chased.
Find a rhythm for 100% charged shots, but don't be afraid to spam if an enemy is near death. Even a short pause takes the shot into the 30-50 damage range.
Don't bother aiming for the head on 150HP characters (Zenyatta, Tracer), as any charged body shot will do.
If you're finding it hard to land shots, go into the options and turn down both your sensitivity and the relative scoped aim sensitivity for Widowmaker (go into her custom controls to do this).
Your poison trap is less about dealing damage and more about alerting you to flanking enemies. Use it as an alarm system, not to kill. On this note, listening for footsteps as Widow is even more important than with other heroes.
Reposition after you get a few kills, because good enemies will countersnipe or hunt you down. Bail out if a Winston or Genji or Reaper comes after you; you are highly likely to lose those fights. Keep moving from your current spot.
Only use your ultimate when it will be useful, i.e. not when the enemy team is respawning or walking back to the point. If someone calls out a flanker in the backfield, use your ult to help people track the threat.