I would just like share with you my experience rooting my SGS3. I'm a first time rooter myself. After using Android phones for 2.5 years, there was one feature that I wanted that at the time I could not get without rooting. That feature was for the keyboard to automatically change to SwiftKey when the phone is in landscape and switch to Swype in portrait.
I did some research and found the procedures to root my phone. The entire process took me about half an hour. As it is my first time, I tried to ensure I followed some rather unfamiliar steps. It wasn't hard, but definitely not trivial. I needed to plug my phone into my PC and move some files around and then do a special boot.
After rooting I installed Keyboard Manager and my phone was able to change keyboards on an orientation change. Since then, I noticed that after a few days, my phone feels a little laggy. It was barely noticeable. However, when playing back music over my car's BT, once in a while, the music stutters. It would happen once every half an hour. Then after another day or 2, it would do it every song. At this point, I have to reboot my phone and then everything would go back to normal. I find that after rooting, I had to reboot my phone twice a week because of this music stuttering.
Last weekend, I found a means to get the keyboard switching functionality without needing to root my phone. I decided to unroot it. I used Kies to do a software update which unroots my phone in the process. For the past week, I have no experienced this stuttering and my phone is as smooth as always. The non-root app I used to switch keyboards doesn't work as well as the rooted app. However, I do find that having a more stable phone is worth the trade-off.
Android phones have many customisation options without needing to root. Rooting involves much more than just installing and configuring an app. If an unrooted phone suits your needs, I don't see too much benefit to rooting. My personal experience with rooting resulted in some draw backs. I know that many people who do root their phone has found it to be more stable or just better overall. Perhaps, more needs to be done to achieve that. I did the minimum amount of work to root my phone as there was only one feature I needed at the time. I suspect that there is a lot more I perhaps could have done, but I had niether the time or the desire to dig deeper into rooting than I did.