If you root and keep the stock rom you won't see much difference really. You will be able to delete bloatware from system directly, which is cool and you'll be able to use ad-blockers and that's about it. If you go for a custom rom you would get some speed and customizations (or not depends on what rom you are flashing) at the expense of stability. Recent devices always work best with the stock rom but there are these little things that you can't change on it.
HTC Charmer, Wizard, Tytn, Diamond, Touch Pro, Diamond2, Desire, Sensation, One X, WP 8S
Samsung galaxy S4
had a few nokias, a siemens and a couple of motos before that