I had the AT&T natural voice working well with V4.5. I evaluated quite a few TTS engines, and felt like the AT&T engine was the most natural sounding.
Unfortunately I cannot get HAL 5.0 to work at all, so I cannot give you any feedback or comments on the AT&T TTS with HAL 5.0. However the TTS engine does work well for a number of other programs I am running such as Cyberbuddy, Natural Voice Reader, etc.
SH