I use the Seidio 1750mAh battery and am happy with it. I've had it for a while so haven't monitored it recently, but not too long ago I know I was getting 14-16+ hours on a charge. This was with normal use for me. No voice calls, plenty of SMS & MMS, lots of Twitter & Facebook, looking things up online, using it as a calendar, to-do list, note pad, Task list, and lots of 'advanced' features as well. I also have my background data set pretty aggressively. Weather updates every 15 or 30 minutes, Twitter and Facebook update every 15, etc. Use the camera a lot, use the LED as a flashlight, and on and on. All that taken into consideration, I would consider my use 'above average', and am happy with 14hrs.
Also, one of the best things I have done to combat 'gotta go my cell phone is dying', is not having bought an extended battery, but having bought a SECOND battery(tho pretty nice having an extended too!). If you have a 2nd, spare battery, keep it charged and with you, most people won't get into a situation where they are without their phone. I depend on my phones for so much, I can't have it die on me. All my appointments are kept within, every bit of info I need to remember, I use it for college extensively-University email, store my documents on it, edit them with Docs To Go, get them and turn them in via FTP, dropbox, blah blah blah. Anyway, so I depend on having my phone available all the time, without exception. I actually have 3 batteries, and if I know I am going to be gone long time, I'll grab a spare or two(I keep both in my computer bag which is usually with me anyway).
Besides for having 3 batteries, I also have many many charger that I have amassed over the years. If you don't some pretty inexpensive ones can be found on Amazon. I think I bought 5 Motorola OEM car chargers for about 1 dollar each. I make sure to usually have a charger or two wherever I go- car charger, wall charger, usb cord, etc. Nice to be able to throw your phone on a charger for 10 or 15 minutes when it's down to 50% or so. Let's face it, even with an extended battery it is pretty easy to get it to drop fast if you need to use it extensively over a short period. Anyway, I've gotten off on a tangent here, so I will digress! A spare battery and a few extra chargers can go a long way. I haven't had my phone die in over a year and everyone else could do the same too. A lot of the time when people talk about extended batteries, they forget that they will now have 2 batteries rather than one, and having a spare, extended or not, can be very helpful!
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