Before anything check your firmwire. If you're running on stock mf3 your bootloader is locked and you need to use safestrap to flash a custom ROM onto your device after rooting it.
As for ROM flashing, yes it's reversible if you make a nandroid back-up. You can recover through Odin/kies as well depending on the situation. Make sure to install a custom recovery on your phone before anything. (Philz, CWM, TWRP are good ones. I use TRWP).
If you're worried about losing some hardware features than just flash a custom TW (touchwiz) ROM that's debloated and more fast and uses the original Samsung framework. I personally think AoSP based ROMS are way more reliable and efficient than TW but you will lose some hardware features. There are many great aosp, GE and tw custom ROMs in the development section.
Disadvantages and advantages are personal opinion. I don't like touchwiz very much at all. I find it to be sluggish, take up too much space, and lack features I find useful in aosp based ROMs.
As for apps after rooted,
Greenify - allows you to hibernate apps to prevent them from running hidden background processes. Great for battery life and performance.
Root browser: ROM features and a great file browser. Can unpack and read .jar,.zip,.apk and many other file extensions.
Trickster mod: If you have a custom kernel this is a nifty tool for CPU/GPU/voltages tweaking.
Sixaxis Controller: Allows you to connect sixaxis controllers via Bluetooth to user them wirelessly (ps3 dual shock 3). Also allows you to make and create custom touch emulation profiles to map buttons to games that don't have control support I.E (modern combat 4, ravensword, order&chaos, etc.)
Titanium backup: allows you to uninstall apps, even system apps and from your carrier. Allows you to make backups of your phone, many other great features.
Sent from my SGH-I337 using XDA Premium HD app
: AT&T SG4 - 1337 ROM: Task650 AoKP KERNEL: KT - MFD - Loki'd
Gaming Governors: CPU: Performance GPU: Performance
Regular Governors: CPU: Interactive GPU: Ondemand
Idle Governors: CPU: Powersave GPU