Yeah I'm not too impressed with battery life (I've had my S3 for a couple of weeks). Thinking about getting that case with the built in backup battery (I forget the name).
I'm not too comfy installing ROMs, every time I go to xda I get confused by all the kernal/ROM talk. There's always a hundred different opinions on which kernals to run with which ROMs, and the tech terms make my head want to explode. Then factor in the CWM/TWRP debate and you can color me lost.
I don't get the widespread Touchwiz hate, the UI is slick and it works fine for me. Sure I could probably free up space by eliminating bloatware, but I just throw all the carrier loaded app icons in a junk folder and forget about them, and I have a 16GB SD card so I've got plenty if space for media, etc. I'm sure the device would also run a bit faster with tweaks, but for a device that fits in my pocket, its plenty fast enough.
Doing the ROM stuff after a while ain't that hard. I've been doing it since I've had an Android device. The downside is trouble shooting and working around bugs. Of course you run the risk of bricking the device (which I have), and then need to know how to recover via software on the computer, and connecting the device in "download" mode. The other downside for me is getting hooked on ROMs. I've flashed and re-flashed ROMs to find the perfect setup.
I use Nova Launcher on every build, and the thing I hated about TW was having to find the code for the "hidden menu" to turn off WiFi power save, which I mentioned earlier in this thread.
Tip: For those who want a quicker response, just go to Developer Options and scale the window animations down to .5. Makes the device seem a lot more responsive.