Am I the only one here? I played through two temples on Ocarina of Time and I didn't lose even one bar of battery power.  
I was using 3D on and off (3D for the boss battles and every time I came up to a part where I thought "This could look cool in 3D"). 
I always hear complaining about the 3DS' "weak" battery life from people who don't own one, but never from people who do. 
 
Am I right, or is everyone hating in silence?

Having brightness cranked up all the way and 3D on constantly can be a bit draining on the battery, but you can change the brightness (there is even a power saving mode) and usually I can get around 8 hours (with the 3D on and off) of play time out of it. When I'm keeping it in sleep mode, my battery never seems to die. I had it unplugged for two days in sleep mode only to come home with one bar gone. I find it weird when people are saying it dies super quick even in sleep mode.
 
Maybe we just got lucky and have awesome batteries or something.

I'm constantly running out.

i agree, I play all the time and the battery seems fine, for me is more the life battery than the time you play

From what I've seen the battery is garbage. There is a reason they gave you a charging dock.

Power saving on, brightness on setting 2, never much problems.
Only time it ran out was when I held a DQ9 play session of about the whole day, but that's a DS game so might be considered cheating.

It's definetly not a problem when I'm playing DS games, and I played quite a lot of OoT in 3D with a low brightness setting (2) without having to worry about it. I do feel like I've had to charge it more often though, which kinda sucks. If you forget , which I tend to do, you'll end up feeling like you can't commit to a longer session. This happened on the DS too sure, but it happends more often here, which is unfortunate.

Lately though, my 3DS battery hasn't run out at all.

I never notice the 3DS's weak battery life... 
 
Because I never play it :(

I'm NEVER running out. The battery is great IMO.

the battery life isn't great, but it's not like i need it to be. i have it on power saving ON and the brightness is usually 2 or 3. i almost never use the 3D unless i'm curious about how something looks. i'm only playing my 3DS for like, half an hour every day and for the rest, it says in sleep mode in my bag/pocket so it works out pretty well.  
 
by the way, when that thing is almost dead, it looks like a fucking bomb about to go off. it's nuts.