I remembered reading in Jeff's 48 Hours with Nintendo 3DS article that he had some concerns about Sleep Mode and battery life, specifically that if you left the 3DS on in Sleep Mode that there wouldn't be much of it. 
 
Or you know, what he actually said, that people probably want more battery life rather than more Miis.
 

WELL GUESS WHAT MOTHER FUCKERS.

Yeah, I've really had no problem with the battery life.  Sleep mode and street pass don't seem to drain it very much at all (though actually finding someone to street pass with in the UK is another issue) and even playing the thing quite regularly, the battery seems to last a decent amount of time. 
 
I do mostly play it at home though, so it's a case of just throwing it onto the dock whenever I'm not using it..  It might be a bit more of an issue if you're someone who really uses it as a portable device but even then, it should last most of the day.

Ive had it on for a full break day (like from 9 AM to 7 PM), then played like 3 hours of street fighter, and it was still at around half battery power. Battery life is fine for my needs!

.

I found the same, people just overreact, as with the red ring of death, yellow ring and system updates breaking the consoles.
I mean, honestly, the 3DS is an ace thing but if you're gaming 8 hours straight on it, you're doing something wrong anyway.

I haven't tested it extensively, but sleep mode does seem to be fine. I haven't had to worry about the battery dying yet.

Seems pretty similar to the old DS battery to me. I street passed at work and played pokemons 3-4 hours before it started to give in.
Speaking of that, flashing red light  to tell you it's about to die is a GREEEEAT addtion.

I have found it to be inconsistent with the amount of time it takes to shut down. I played a game for 2/12 hours from full charge and it completely died, I played the same game  another time and after 3 hours and the battery was half empty, same settings both times. I've had it run for ages in sleep mode, almost 7 hours.

" I found the same, people just overreact, as with the red ring of death, yellow ring and system updates breaking the consoles.I mean, honestly, the 3DS is an ace thing but if you're gaming 8 hours straight on it, you're doing something wrong anyway. "

Having my 3DS in sleep mode from about 7:00am to 3:30pm with about 1&1/2 hours of SSFIV in-between, I still had about 3/4s battery. That's alright for me.

I just want to say that a portable system is often like any other gaming system for me. I can play for several hours straight, as long as I find it entertaining. Implying that portable systems only are made for bus rides is nonsense to me.

" @TaliciaDragonsong said:

" I found the same, people just overreact, as with the red ring of death, yellow ring and system updates breaking the consoles.I mean, honestly, the 3DS is an ace thing but if you're gaming 8 hours straight on it, you're doing something wrong anyway. "

That's not really why people are concerned... the concern comes from when you're somewhere and can't charge your 3DS for the ~3 hours it needs... you know, what with it being a portable device and all, that you take to places. "

I've had mine on for ~12 hours today and it's still at 3/4 battery. This is with 2 hours of gameplay, wireless and Streetpass on.

Sleep mode is just fine.  I've gone out walking around with it on a couple of occasions for hours at a time, and the battery was barely drained.  (Also, Play Coins.  Yeah, baby.)

" Sleep mode is just fine.  I've gone out walking around with it on a couple of occasions for hours at a time, and the battery was barely drained.  (Also, Play Coins.  Yeah, baby.) "

I think the battery complaints are vastly exaggerated.  
 
Also, don't put it at full brightness all the time.. 

i was always wondering if the battery was crap or not w/ sleep mode. i might actually carry it with me now

Mine last me all day in my bag street pass enabled. From 6am till about 9pm when I get home. Most days I still have over half a battery left. I bought an extra adapter just in case to keep with me and haven't used it yet.
I play alot of DS games in it though and they seem to use less battery. I could be wrong about that.

Ok so I played Super Smash Bros for a while until I had 3 bars of Battery Life left and then I put it on sleep mode for the night and in the morning I check on it and its dead so I go charge it.

I think that maybe it will last in sleep mode for the night if you have 4 bars of battery left not 3. Oh well