Will Eclipse ADT be deprecated when Android studio reach its first
stable version?
No. According to Xavier Ducrohet in this keynote the Android team will maintain the work on the ADT (At 00:35).
Why the Android Team started another IDE instead of continue improving
the old good ADT ?
I'm paraphrasing Xavier Ducrohet now: He says that by building your own IDE they can improve the workflow for developing Android apps. And to be honest, in my opinion the current version of IntelliJ is a lot faster than Eclipse Helios. You just have a great overall experience by using IntelliJ. This is a matter of personal preference of course.
To be honest I feel very comfortable with eclipse and find it much
more simple/clean/user-friendly that Android Studio or intellij. For
me I will continue develop with Eclipse, is this a good decision?
Maybe. You shouldn't have to switch, if you don't want to. Like I said before, they will still be maintaining the work on the ADT plugin for Eclipse. But at least give IntelliJ a try. I'm sure you'll be very impressed by its performance.
Update
At this point, I would strongly advise everyone to use Android Studio for android development. Although the ADT plugin for Eclipse is still available, support for it is lacking and the android developer tools team has shifted their focus almost entirely to Android Studio. And quite frankly, Android Studio is an amazing IDE, you should definitely switch, if you haven't already.
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