I’ve just begun using Gravity Forms but I’ve used Formidable quite a lot. My biggest complaints about Formidable is the way it handles upgrades. If you have a ‘Pro’ account, when an upgrade is available you have to go through a multi-step, not very intuitive upgrade process every time.
Other than that, so far Formidable has worked very well for me. It doesn’t have all the ‘extras’ that Gravity Forms seems to offer, like MailChimp integration, but that hasn’t been that big a deal. And, it’s far cheaper in the long term with a one-time, unlimited license for $99 (when I bought). Gravity Forms makes you renew every year or lose support.