From what I understand a framework reduces complexity in areas that are common, like a log-in system. I use ASP.NET MVC at work and have done some work in Zend Framework but do not get how a framework helps with client side development. The reason ASP.NET MVC is used at work is for Unit Testing - does a Flex framework help with this too?
Please let me know why I should or should not use a framework with Flex?
If you've ever tried building a slightly large application, or one at all complex, things can quickly get out of control. I don't know how many projects I bailed on when I was first starting out because I didn't know patterns, or how to make parts of the system communicate without being tied to each other, or dependent on each other.
So, basically a framework is a collection of patterns put together. Theoretically, if you learn to follow the "rules" of a (tried and true) framework, your app will not get out of control to the point where you find yourself fixing one bug and causing two. I've been there and it's not fun.
I also found that by learning to use a framework you initially don't have to know so much about the patterns behind what you're doing. But before long you'll get a good handle on the patterns used and you'll be able to apply them in new situations or find a better pattern. So it's a good learning tool as well.
I'm sure people will have arguments against using frameworks -- this is just my experience. But if you become familiar with a few, you will likely find that one might be suitable for one project but not another.
As far as Flex frameworks, I personally like PureMVC. In all honesty the only other one I've given a good amount of time to is Cairngorm. But I like PureMVC because it feels right to me, but also it's generally not dependent on so much on the built-in Actionscript classes. For example, it uses it's own notification system. So if the notifications change in Flex, they'll still work in your PureMVC app. Also, the creator Cliff is super helpful in his forums, and he's really passionate about it. And the documentation is great.
I recommend coming up with a super basic app, and building it without any frameworks, then again with a couple others. You don't have to finish the app, but just get a feel for what's behind the frameworks.
The short answer is: it depends on the framework. :) My thoughts below:
Flex is itself a framework, and you can write reasonable applications without the need for any additional framework. Flash has a built-in event model that allows for bubbling events, so you can dispatch an event in a deeply-nested user interface component and have a listener higher up in the hierarchy that handles the event. The event handler can delegate to your model, which retrieves data from the server, and Flex's binding support can ensure that your views are appropriately updated from the model. I think it's important to understand that Flex applications can and should be written more or less according to that approach, and that any additional framework should help facilitate that approach, rather than providing its own way of doing things that ends up coupling you to the framework.
That being said, an extra framework that helps facilitate this approach can absolutely provide value. I would recommend Mate or Swiz because I think they achieve this goal. They don't try to reinvent the wheel or replace parts of the Flash / Flex APIs; instead they complement them. The dependency injection features make it much easier to provide data to your views, but without coupling them to any framework. There are a number of utilities available to make it easier to work with remote services. They also have a utils to facilitate testing or even persisting data in shared objects.
I have also worked with Cairngorm in the past and I would not recommend it. CG is notorious for requiring you to create a ton of classes that adhere to CG-specific APIs and requiring you to use many of their Singleton implementations which make your app brittle and hard to test in isolation. It is based on a number of J2EE patterns which fell out of favor in the Java community at least 5 years ago.
I've read a little about PureMVC and while I can't speak to its invasive nature, I think that reinventing the event model (called "notifications") is silly and couples you to their framework. Sure, you can say it "insulates" you from the Flash event model in case it changes, but I'd say the odds of PureMVC changing their notification model is far more likely than Adobe changing the event model. :)
You may find value in using a framework if:
Here's a great article on Flex frameworks. Flex Framework Comparison
And, I agree with the conclusion...Mate is a great Flex Framework.
Another interesting framework not mentioned in this article is Spicefactory's Parsley.
I use PureMVC (which is an MVC-framework for Flex) for my larger applications. Like 99miles stated, it depends on the application size.
If you just have to make a (relatively) small tool/application, you can get it going with just Flex. But if you want to create a large, complex application a Framework might come in handy.
The main reason I use PureMVC in Flex is because of the Model-View-Controller architecture. This separates the presentation layer from the business layer and works a lot with events and notifications. This gives you a lot of control in creating reusable classes/controls in Flex in a relatively simple way.
As I said before, for a small tool/app/website I wouldn't recommend using a framework, because in that case it might be to time consuming and complex. But for the larger applications, frameworks are quite handy.
More information on the MVC architecture can be found here. Hope this helps :)