I find my phone to be extremely stable. Are you sure your friend wasnt saying that if you were to put 3rd party apps onto the phone it would become unstable?
Yes, mine is stable with 3rd party apps installed (knocks on wood). It depends on the apps though, Apollo isn't stable and forced me to restart so I'm using BeeJive/Meebo now. Navizon is fairly stable but when zooming in too far it quits out. I haven't tried any other apps besides SMBPref, Summerboard and Services. Those have been stable so far.
No actually they were saying that apple is not allowing third party apps because the os is unstable.
My unlocked iphone with third party apps is extremely stable. Sometimes Apple apps AND third party apps crash just like they do in Macos x, but the core OS seems very stable to me.