DHD users here with Revolution HD 3.2 custom ROM and radio flashed.

On GSM mode all the time, making around minimum 2 hours a day due to my work. Bluetooth on all the time, paired with headset.

9am (usually start with 90% battery), usually left around 40% at 6pm.
Not too bad if you ask me.

Been using Xperia x1 before this on similiar usage, not much different in term of battery usage.

IS can be better but DHD may not loose far, certainly not just half day as somebody mentioned.
Unless gaming non-stop, there won't even last 4 hours on DHD if u ask me.