downside is they're more expensive than their steel cousins. The goodside is that they're usually stronger than a factory unit, due to their larger diameter and thicker walls, and they usually result in a better ET at the dragstrip, usually around .05 second minimum compared to a average steel driveline. You might be able to get more if you have a real heavy steel shaft now and switch to a aluminum one.
I just replaced the stocker steel on my '69 and noticed a big difference in vibration. I know my u-joints were good but maybe my old driveshaft was out of balance. I got the new aluminum from Mustangs Plus/Inland Empire driveline. Well worth the $$.