Hi guys, big frustration:
I'd like to install some apps present in extras-devel (or -testing) repository but I know is dangerous to do that. Almost everyone advices that is better to disable its! but...
For example I'd like to install "man" command for terminal and it's in -devel, so I wrote an email to developer (Mohammad Abu-Garbeyyeh) asking when this app will pass in extras and he replied me:

Now, I believe him and I'm going to install "man" from -devel (he told me that and he's the mantainer so...why don't believe him?)
but this means...how many packages we will never see in extras? How many apps now in -devel we could install knowing its are sure?
So...from that my question: how to know which apps are sure in extras-devel and extras-testing so we can install its without risk for our n900 (block/must reflash/not optified/etc...) and use its instead to wait and hope its will pass to extras (maybe never like "man" package)?

very simply:
try and error

you could always compare package lists with:

http://maemo.org/packages/

(look at the links below textbox)

extras means tested apps, extras-testing means apps the developer thinks are ready to extras (and fulfill the criteria) and extras-devel could be anything. so there is no way knowing that an app behaves perfectly unless not to use testing or devel repositories. you could always contact the creator/maintainer of the package and ask. but keep in mind that if developer has tested the app, it could still mess your device up (and thats why extras require package being tested by multiple persons)

e: feel free to comment to package page if you install devel app and it doesn't cause any problems etc. then others can see your comment and have at least a little information about the app.

nuooooooo!! please atilla...it's a very unscientific method!!!

Extremely happy to do that as my small contribute to this great community!!!

haha.
seriously:
ossipenia is totally right and everyone here will say that too.
you can't know the status of an app in testings/devel.
be sure that you know how to flash,the best would be if you flash one time to know how compatible your pc with the flasher suite is.
if you have/can do these things don't be shy and just try it.
but never forget: devel/testings can harm your device do it on your own risk!!!!

Technically, none.

If a developer puts something in -devel, and he tells you that "he wont put it in -testing" this means that no one is actually testing that it wont put your phone in fire

So, if you trust the developer, go on. But if even the developer tells you that "it won't probably pass the criteria" so dont worth the time to do the -testing procedure, use your common sense!

There are no sure place in extras-devel

On the other hand, if you're willing to live on the edge, feel the wind ruffle your hair and aren't wearing a turtle neck, know that some of us has enabled -devel and left it on!

Extras: needs votes. Popular applications here.
Testing: needs confidence. When stuff is moved there, is dependent on the personality of the developer.
Development: has the most software.

If you're willing to brave the risk of installing something that isn't ready, or unstable software replacing stable software, the big bads doesn't really happen that often. Stuff will uninstall, sometimes.