Speed is a pretty fair ally with a high floor but low ceiling. With 3 thwarts of 2 and a block, you get exactly the value you expect for a 5-ER ally. The response lets you front-load with 2 thwarts right away, which could target different schemes, and you can clear his ally slot sooner to make way for a new ally if you're full. I've also liked Speed with Earth's Mightiest Heroes since it gives you another use of Speed's readiness on the turn where you only thwart with him once. Not a staple of Justice I would say, but a totally fine card to use.
Attack: 1. Thwart: 2.
Response: After Speed thwarts, ready him. (Limit once per round.)
"Hey! Geriatrics! Let's get a move on. The Avengers need us!"
Scarlet Witch #10.