My n900 reboots in several minutes when overclocked to 805MHz and smartreflex enabled. It seems more stable at stock frequences.

Power savings are similar to what undervoltage provedes. If I remember correctly, under full cpu load current consumtion was about 250mA when only undervolted and something like 240mA with stock voltages and smartreflex.

And about 805MHz... With "ideal" undervoltage settings processor under full load consumes just a bit more than at stock settings 600MHz, so it seems quite safe.

N900 should last for 4-9 hours of continuos usage or 2-3 days when left alone. If your battery drains faster, that's not normal...