Momentum is your ally

It's always harder to restart an engine than to just keep it rolling along.

So don't stop. 

With iterative development, you're always delivering frequently and the feedback loop is short. So what do you lose if you keep going? 

From experience, when you stop a bus, people hop off, go wandering and it's like herding cats to get everyone back on board.

By all means, take a breather, re-adjust your course based on what you've learn't, but don't. ever. stop.

Most of the time the person that recommends you to stop, form a steering committee, and re-establish the project plan is probably the person that that needs to be kicked off the bus.