It is the most commonly used Framework for Flex Applications. If you're a consultant, I recommend learning it.
If you're doing your own stuff, I'm not sure what your best route will be. I'd try to learn enough to either be interested in moving further or to decide it isn't worth you time.
It's a very... boilerplate architecture. It's good to know but your milage may vary. Some might say it's better if you're working on somthing large and complex, but then again, that also multiplies the boilerplate nonsense. Some may say it's good for beginners... and that may be true but probably just because you'll learn it.
I don't think it's "actively developed" anymore, but it is certainly heavily used. It's actually a good thing that it's not "active", it shows that it's mature enough that it doesn't need more crap added to fix previous flaws or shortcomings. Open source "frameworks" that release new versions all the time usually turn into a huge pile of crap as they try to solve every problem imaginable and do all the work for the developer. Cairn is probably the most mature Flex architecture, if there is one. The important thing is that it is "updated" for each version of Flex, but I've yet to hear any complaints about a new version breaking it (it's not that complex under the hood).
Personally I've moved on to trying out Swiz. But it's not a heavily used architecture, and it may not be the best for beginners either due to the amount of flexibility it allows you. It leaves more choices up to the developer and only provides useful helper tools for getting your job done. But as far as simplicity and avoidance of useless boilerplate code, Swiz can't be beat.
Another popular upcoming framework/architecture is Mate. I didn't look into it as much as Swiz but it seemed to have more boilerplate than I really wanted, especially after using Cairngorm. But from what I hear it has more functionality built into it than Swiz and a lot of people seem to like it.
I would say if you're working on a small, beginning project that you can complete quickly (or even back out and use a different architecture if you want), Cairngorm may be a good first choice. But there is definitely a lot of new competition out there for it, so it may be on the way out now. It's hard to gauge such things.
Also, I forgot to mention, I think there's a code generation tool out there for Cairngorm. This to me is a bad sign, though. Why should we need to generate all this code? Isn't there a less verbose way? Sure, it solves the problem of having to write (i.e. copy/paste -- this is terrible) boilerplate, but at the end of the day you still have a crap load of files and hundreds or thousands of extra lines of code to maintain.
Thanks a lot for u’re input, that was very helpful
I done some research too, and lot of people say that cairngrom is good and mature but that is a bit overloaded with all that j2ee-like patterns structure, which is “as they” say not needed in whole in RIA. Because I have not experiences in RIA architecture I wondering what “mvc-kind” framework could be the best?
I've seen both of those. Puremvc I avoid like the plague, maybe it works out for some but it's even more boilerplate and heavyweight than Cairngorm. The puremvc website claims it's "lightweight" (which makes me laugh) but compared to ALL the other Flex archtectures out there, it seems like the most bulky and involved out of them all! (Lots of things claim to be "lightweight" nowadays but that doesn't make it so.)
Penne I'm suprised to hear mentioned, it doesn't seem very well known or used. (But perhaps I'm wrong or outdated here.) When I tried it, it seemed a slight improvement over Cairngorm, but I think there was still boilerplate code and the examples and "best practices" information out there are even more non-existent than Cairngorm's. (Of course lack of examples and best practices with these things isn't uncommon.)
Tony Hillerson did a nice comparative evaluation of a few "frameworks":
He settled with Mate, but based on his same evaluation I looked into Swiz.
IMO in recent years the whole J2EE scene has turned into an obnoxious kludge of various hacks and crap built onto crap meant to "improve on" flaws or shortcomings for some other crap, on and on in a seemingly endless process. (The more automatic, the better! Java devs seem to want to do development without actually having to DO any work, but you can't get something for nothing so it's a hopeless quest.) So I would agree that implementing Java style MVC within Flex is overkill. But then again, I am a bit of a dissident in the Java world.
So yeah, Cairngorm is still a decent choice to start with, and I think I read that they were working on some kind of best practice resources. But I'm still determined to see how Swiz works out because I'm just totally fed up with boilerplate and complexity. Simplicity is too often forgotten about in the software world.