VoiceStream and Cingular are two completely different companies/entities/networks. Sure, they both do GSM 1900, but that's it. Where you live or use the phone will dictate whose network you're using. There are surely some areas where both VoiceStream and Cingular are licensed, and in those areas, you're on on whoever's network you signed up with.

So, whether or not Cingular is any good has no bearing on whether or not VoiceStream is any good. Also, that's nothing specific about the Treo either. Plenty of people used phones with VoiceStream (and Cingular) long before the Treo came around.

If your point is that a purchaser of a Treo has to use it on a GSM1900 network and Cingular and VoiceStream both suck (which may or may not be true), then the Treo does come at a disadvantage since Handspring only activates it with a VoiceStream or Cingular service plan. However, there are other GSM1900 carriers in the US, and a simple SIM card swap will make the Treo work with them. Presumably Handspring will offer other carriers' plans eventually.

(OBTWIAVBP: Discussions of GSM900 and GSM1800 (both of which the Treo also does), and the rest of the world outside the US is completely elminated from this discussion for the sake of brevity)