It's hard to see a down side to the technology other than its requirement that your graphics card and chipset both come from Nvidia, which is understandable given how the two must cooperate to pass output duties back and forth.

Unfortunately, it seems that HybridPower has no future on the desktop. The new GeForce GTX 285 and 295 don't support it, for example, and neither will the new desktop GPUs that Nvidia has in the pipeline. According to Senior Technical Marketing Manager Sean Cleveland, HybridPower has been dropped from desktop parts because, thanks to the improved idle power consumption of Nvidia's latest graphics parts, it no longer "substantially improves the user's experience." HybridPower will live on in Nvidia's mobile platforms, where the company still sees merit in the technology's ability to reduce heat output and prolong battery life.
http://techreport.com/discussions.x/16604

So much fuzz about this at Cebit 2008, we already could guess something was up that we hadn't heard anything more about this since then