I had a Zero Lemon slim extended battery on my SGS3. It was 2400mah vs the stock 2100. It was pretty good, giving an extra 30-45 minutes or so extra screen time and another 90-120 minutes of run time. If you find you're just cutting it close at the end of the day and you can't charge mid-day, it's worth the $20 for a pair of them plus a charger. You can get them on Amazon too. I haven't bought the S4 versions yet but I plan to - my wife and I can each get a little more battery life for a total of $20 that way.
You lose NFC, though. that's probably why they have a higher capacity - no NFC antenna. If you use that feature, or S-Beam or whatever, skip the Zero Lemons.