There has been WAY to many 40D Live View threads and not enough searches and manual reading. This had lead to me reading WAY to many threads with false info in them.
First Off......You can auto focus in Live View Mode as well as have shooting info on the screen.
The index of your manual (Chapter 6 pgs. 107-114) will show where to find this info as well. (sorry for you overseas chaps as Canon seems to think you don't need to have manuals included with your purchases)
1. How to turn on LV (pg. 107) - shows how to turn on LV and start/stop it.
2. C.Fn III-6 - Enable (pg. 160) - shows how to Auto Focus with LV on. (ps...push AF-ON button & One Shot must be set, mirror will lock up and focus on center point (beep) and then drop to show LV in forcus)
3. C.Fn IV-7 - LV Exposure Simulation (pg. 163) - show real time brightness with LV on.
BONUS - Info on LV screen - (pg. 112) - Press INFO button while LV is on 1 push gives you shooting info. 2 Pushes gives you all of 1 push + style mode and battery life. RGB Histogram is also possible with C.Fn IV-7 enabled.
BONUS - (pg. 110-111) With LV on, you can zoom in 5X and 10X by hitting the magnifying glass button once or twice. This is very useful for manual focusing on an object.
BONUS - With LV on, the joy stick will allow you to set focus box off center (thanks CyberPet) Center Box will also show exposure as it moves (thanks BogongBreeze)
BONUS - With LV on, DOF button works in LV. This is great is you have C.Fn IV-7 disabled. (thanks BogongBreeze)
BONUS - (pg. 114) With LV on, AE Lock and FE Lock button works
BONUS - (pg. 107) LV can be use "live" via a computer connection with supplied Canon Software.
BONUS - LV works with TC-80N3 / RS-80N3 / 60 T3 cable switches
BONUS - SILENT MODE - (pg. 106 & 113) Discusses the 3 Silent modes (flash and fps) for LV. This is important for non-Canon off shoe flash firing.
BONUS - LV will automatically turn off with what ever setting you have set for Auto Power Off (pg. 42)
I would also suggest you turn Lens IS off when using tripod mounted shots with LV.
I would also suggest you cover the viewfinder to get proper metering when tripod mounted.
-Hope this clears up the 40D LV mystery and the manual reading problems (again Chapter 6 pgs. 107-114).
Is LV useful? For me YES. I use it for tripod mounted landscape shots, odd angle shots, tripod mounted product shots, and tripod mounted macro shots