The contact/call history back up really is overkill for me as I back those up to SD, and the contacts sync up with Google, but, meh, I do it once a month or so.
The remote locate is more important to me. It's pretty cool for long range or if the phone ever goes missing, and the scream is pretty cool. I fidget with my phone so much that I can't imagine not realizing it fell out of my pocket at a movie or restaurant, but I can imagine using it one day, and it's the kind of feature you want BEFORE you need it.
Just an hour or two ago, though, I got an e-mail for signing up for a buck for a year of a "premium" program (sure a money grab, but I tip a buck when I buy coffee, so what the heck). The first premium preview feature is dead awesome. It's a remote lock that survives reboots and requires going to the web site to unlock it again. You can't unlock it on the phone, unless you get someone who knows how to and is willing to boot into recovery to restore a backup. Pretty sweet for the 95% case when your phone fell out of your pocket at a restaurant or the like and you want to lock it up until you can get to it. Nothing's perfect, but I like the other features on the app.
So, it's not a must have app, but it's one I want.
Oh, and for those worried about "battery sucking" I've never seen it on the top 20 users of battery any time I've checked to see what's using battery on my phone. Juice Defender and Juice Plotter both use way more battery than Lookout.