Hello. I am going to buy Desire Z and Aria next week. I was thinking why people root these phones? Aren't these phones good enough ? Do you recommend me to do it ( but i have guarantee for my phones) and how can it help ( since the original Android system is very easy to work with )?
Thanks.
Most probably you don't need rooting, except you need custom firmware.
I own a desire Z too. I recommend it to any android fan.
I'm currently running the 2.2.1 android version. I didn't root the phone and i'm not planning to do so anytime soon. I'll just stick to the official releases of system updates.

But I also keep myself updated about the latest updates for my phone. and yesterday, i came across a link (http://www.androidcentral.com/gingerbread-rom-htc-desire-z-leaks-out that shows a leaked Gingerbread ROM for the HTC Desire Z.

It's up to you. For me, i'd rather stay on the safe side.
And btw, good choice dany again. HTC devices are simply super-cool.
Does the 2.2 Android Rom change your menu (home ) screen like Nexus One phone with the dialler and 2 buttons or the 7 original homescreens? Or you can choose between them ?

And what is the difference between rooting and custom firmware ?
rooting will give you access to everything in your phone,custom f.w is an unofficial f.w for your device

ex:the htc G1 had an official android version 1.6(?)
but some modders managed to run it on 2.2! :) (un-officially)