Currently running a level 10 Mastermind.

I don’t have much experience as an assassin but I went for a well rounded build, using either a crossbow at range or a rapier/ dagger in close quarters.

He is good at traps, spotting hazards, and social interaction. Not too shabby at lore checks either.

It feels like there is always something I can do, and he is so maneuveable.
The tactics angle and social skills pushed him into a party leader role.

I love it.
I have really enjoyed how dynamic Rogues can be in this edition. Once you hit level 11, and if you have picked up some extra proficiencies along the way, there is so much you can do outside combat.

In battle, you can go wherever you want, avoid enemies, and pursue side objectives at your leisure, or pick a target and put on a pretty good level of hurt without expending resources.

Rogues are fun.