Just wanted to chime in...... I ordered a Seidio 3500 mAh, got it on Tuesday. Installed it and started charging it Tuesday night so I could get the recommended 8 hours of charging in. It's been unplugged since 8 AM Wednesday morning. Currently at 3 PM my battery is at 33% (Battery Left is still calibrating, currently with moderate accuracy - says I should still have about 12 hours of battery life left to go, estimated 40 hours 42 minutes of full battery life).
So far it's mainly been a mix of moderate use and some heavy use. With my stock battery, I would usually be hitting about 20% battery left by 6 PM - and if I did any sort of heavy use, I could count on my battery dying around then.
As others have mentioned, if this holds up, I think it will definitely change the way I use my phone - I was definitely getting to a point where I was nervous to use just about anything (turning on 4G would kill my battery, for example). I don't actually need 40 hours of battery life, but I have a feeling this definitely means I could get in a full day's worth of much heavier use than I've been willing to do, and opens the door to using stuff like tethering more now too.
The extra thickness and weight does not seem that bad to me, and the tradeoff of essentially not having to think about the battery definitely seems worth it.
I will probably keep charging in the phone (the process of taking the battery cover off seems just nerve-wracking enough that I'd prefer to avoid doing it when possible). I am glad to see the explanation for why the initial battery life seems to drop (it's been bugging me that within 30 or so minutes after unplugging my phone I'd see it drop 5-10%, and then it would settle down).