So far running with 512Hz mod on my both N900 and no issues with battery consumption or any other areas. Other device has been on idle (still in connection to network) last 4 days and there is still 27% battery left. Pretty OK I would say.

What comes to usability. I run tests with 'pi_css5-vfp3' (pii calculation test sw) and results between 128 and 512 were identical. So no power increment gained, but overall responsiveness is a bit improved. Programs starts a little bit faster. I did comparison between my N900's, one running with 128Hz and other with 512Hz. 512Hz was always faster (applications up and running). Both devices are using identical sw settings and images (back up from one to other). So I have to disagree with comments about that no visible changes observed.

Edit: Performance delta between 128 vs 512 is not very big, so difference is not so easy to observe. Comparing when devices are side by side is giving this evidence that 512 is a bit more snappier.