Ripped straight from http://haruhichan.com/wpblog/?p=205.
Pros of Hi10P:
Significantly higher compression ratios resulting in vastly decreased file sizes. The first rounds of testing show a 20-40% improvement on file sizes
Far more information is preserved from the original, removing such issues as banding and poor detail in dark scenes
The encoder will have less things to worry about
Cons:
Slower decoding and encoding
No support for DXVA or CUDA as of writing
Slight compatibility issues when using obscure, obsolete devices like the PS3 to play back movies — but who does that, really?
I'd say 10bit is worth it alone for the smaller file sizes and better visual quality, especially debanding being a very good advantage. Here's an example (theoretical, but there are actual 8 bit vs. 10 bit comparisons out there; just can't seem to find the link right now):
8bit
10bit
The differences are subtle (look at his red shirt and the banding on it), but if it's better visuals at a lower file size, I'm all over it. And, as for people complaining about it stuttering or lagging, then it's about time for an upgrade (although there are cases where CPU usage seems to shoot up dramatically even on high end machines; I currently think that's a codec problem or a combination of codecs interfering with each other and not the machine itself).