Yeah definitely, we used Pivotal Tracker for a long time ourselves. There's a lot to be gained, particularly from a scheduling and reporting perspective, by having estimates but without the strict sprints of Pivotal.
On this topic, it would be great to have some kind of gantt chart UI for scheduled cards.
not exactly estimating, but I built something to let the team vote on items in a Trello list. We usually vote more along the lines of "bang for the buck" ie importance/estimation http://blog.forcerank.it/tame-your-trello-backlog