We use part of Zend MVC (the dispatcher, controllers, and view scripts) here and a lot of the other facilities such as the autoloader, config, etc. and are very happy so far. As long as you design your application with an eye toward portability, you won't be tied to ZF. For example, put all of your business logic in model classes instead of the controllers themselves. That way if you ever need to move to a new presentation layer or use the business logic outside it (e.g. in SOAP or RPC messages), you'll be ready.