ICS is real smooth and offers unbelievable battery life, because of how 4.0 handles 'standby' battery power. So you'll immediately see improved battery life. Case in point, my D2 on 2.3.6 was getting 8-10 hours a day on a charge; after CM9 (and Galnet Miui) were flashed, I EASILY got 24 hours on the same use. Granted, that was a CDMA and not an LTE phone, so LTE will use more juice, but the savings will be noticeable.
Of course, LG could completely narse the whole thing up and we could have a repeat of the OS we currently have for stock, but I'm hopeful they got their shit together in the 6 months since the phone's launch.