Re: Custom Rom
I couldn't begin to tell you how many ROMs I have flashed. I have tried almost all the major ones: CM, AOKP, Slim, Liquid, Carbon, on top of stock modified for my S3. Each one will probably have bugs/quirks that you don't like.
Regarding my M8 (and many of these things are applicable to ONLY the M8):
For me, Liquid is a really nice ROM. The double tap/swipe to wake my device is works about 80% of the time. The near daily UI crashes are what caused me to move to a different ROM. Carbon has some sound bug issues. I don't care for Slim's restyled Recent applications menu (personal distaste; the menu at least works). CM has some double tap to wake issues, and rotating issues as well. I haven't had a chance to try AOKP on my M8 yet. I might pretty soon.
To summaraize: CM is fast to update (mind you that is not always a good thing), Slim's releases are very stable but slower, Carbon is a balance between those two. Liquid was a very smooth running ROM. But when the UI crashes it becomes unresponsive for a few minutes (once the crash is over with the device is fine for another day or two). The crash sounds a lot worse than it is. The device becomes unresponsive (by now you know it is going to happen), then, where ever you are you are pulled to your homescreen, and if you have a PIN password set up, you reenter it and you are in. All your apps are fine, and you don't even lose your place.
You might have different results. App A might work but App B might crash constantly (this happens with Google Play Sound search on CM and I think Carbon for me); so it depends on what you can live with. Up until a few weeks ago, I ran Liquid since June (I was on Carbon from May to June). And believe it or not, I am seriously considering going back to Liquid.
I think what Rukbat was getting at is this: Why do YOU want to get a custom ROM?
Keep in mind.....rooting your device can void your OS warranty. Messing around with ROMs when you don't exactly know what you are doing can lead to bricked devices. I would strenuously suggest not only looking up how to root, but how to unroot and common problems in the rooting process, and how to fix them. I am NOT saying that I don't want to help, but it is better to be self sufficient when it comes to matters like this.