Is Aero the most important thing?
Although I would like to enable Aero as well, I think the feature is way overrated. For whatever reason, Microsoft implemented it so badly that it requires newer hardware to even run. Compare this with Leopard that was still able to run on legacy PowerPC machines just fine. My guess is that getting Aero to run in a VM environment such that it makes the Microsoft's WEI roadblock engine happy is no small feat. Understand that I'm not trying to perpetuate a "bash Vista" mentality, but when all is said and done, do you really need Aero to comfortably use Vista? For folks who develop for and evaluate software targeted for Vista, I can clearly see this as an issue, but for others, it's probably more of a "nice to have" feature. I run a Vista VM and, while I would "like to have Aero," I "don't actually need Aero."
Having said all this, the new features of Parallels 4.0 are quite nice. The ability to utilize the multiple cores of the host processor is particularly a welcome addition. I've been using the software throughout the beta testing cycle and have not been disappointed.
You folks need to make your own decisions, of course, but don't overlook the other features of 4.0 just because they don't have Aero yet.