I've only flown them once (someone else booked it), and all went smoothly. However, from reports from friends and colleagues, I gather than when it works its fine, but when things go wrong they go horribly wrong.

Make sure you check all other possible airlines to make sure they really are the cheapest (by a considerable margin) first. And if you don't like huge check-in queues, forget it altogether!

I would never fly EJ on business, but might consider it on vacation if it offered a considerable cost saving...