[Galaxy S 4 Q&A, Help & Troubleshooting] BigJim.T32:
For me, custom ROMs nearly always beat stock roms, hands-down, and this has held true from my old ZTE blade (Orange San Francisco), two Galaxy S2s, and my S4, probably down to the greater level of user-input that goes into them, rather than the design-by-committee process that OEMs use to try & impose their brand over the vanilla Android os.

I only moved to this S4 (i9505) within the past month or so, so the experience is still quite fresh in my mind...

I agree with much of the advice of the "try it and see" variety, but I also understand that for some of us, we only have the one phone, so when you're using it as your daily device, you don't always fancy going on a magical mystery tour of trying out multiple roms, if it's likely you're just going to go back to the rom you got rid of, and have a massive task of putting everything back the way it was, having wasted a day of experimenting... :p

1) I hated the bloat that came with the Sammy rom - sure, there's a tool (debloater) that can block certain apps from running, and if rooted, can remove them from the phone - along with a "safe-list" of apps that you can safely remove without breaking any features of the phone.

The bloat isn't just a problem for OCD-esque reasons, these apps take up space, they can run at inopportune times, update themselves using your data, eat up clock time and generally get in the way of having a device that only does what you want it to, when you want it.

Most custom roms tend to give you a (mainly) clean slate to work from, allowing you to pick & choose the functionality you want by installing apps manually - so (like me for example) if you only want to access gmail through your web browser (just like you would on a PC), and don't want a half decade's worth of emails syncing between your device memory & the web, you don't end up with multiple email clients on the handset all clamoring to be by synced 24hrs a day...

Battery life is a MAJOR factor here - I saw a big improvement after debloating the stock rom, and even better since going to CM12.1....

2) Cyanogen (like most custom roms) give more options regarding features that the stock roms bake-in.

For example, heads-up notifications are screwy in the Sammy roms - I'd get a notification of an SMS received in the notification bar. I'd drag-down, double-tap it, but then I'd have to manually un-lock the screen before the SMS app popped-up and displayed the message. Some people hate heads-up notifications full-stop.

Seemingly minor features like these, that after a while begin to really bug you are often controllable in custom roms like CM, or just plain old implemented better, after real-life user input and advice has been given.

3) User control; It's silly how much additional control you can gain from a well thought out custom rom, and the popular ones tend to be well thought out (as they garner a lot of user-input).

For example, in CM you have a additional control over root access in the device settings, fine-tune the interface and gives more fine control over many aspects of the device that if you're used to the Sammy rom you'd think can't actually be accessed...

As to what you'd give up - other users have pointed that out.

All I can say is I'm quite demanding and I've found nothing that I can't do if I need it - for me it has only been a positive.

Some people say the CM camera is less sophisticated than the Sammy one - I personally prefer the simplicity; There's an abundance of high-end camera apps for those times you want to take a studio quality photo, with detailed light readings, composition aids, etc... But mostly when you're taking a quick phone-cam shot, the ease of use is the main factor (and who doesn't tweak their photos at home with photoshop anyhow?)

So a simplecam as the default and the option to go high-end later is for me, the better way round....

I know some of this has strayed slightly off the specific questions you asked, but having made the same change you're thinking of in the past few weeks, I thought that giving some food for thought might be appreciated... Hope it helps... :)