That's exactly the same position that I'm in. I've rooted my EVO and am constantly flashing different ROMs and Kernals to find the ultimate combination.
Rooting your phone is basically like jailbraking an iphone. It unlocks the phone, if you will, to give you full control of the device rather than the limited control that you have out of the box. If nothing else, it at least allows you to delete all of the bloatware (bullshit sprint/verizon apps that are on your phone that you don't ever use) off of your phone to free up that memory.
As mentioned, you can flash different ROMs (think interface) to find one with the features and setup that you'd like to use. You can flash different Kernals on the EVO to give you greater speed/battery life/ etc. (or worse depending on the combo) However, I don't believe you can flash kernals with the X.
Don't like the keyboard on the X? No problem - you can change it. With a rooted phone, you have full control over the files on the device so you can whatever you want to it. It should be noted that this can get dangerous if you don't know what you're doing, so please educate yourself first so you don't brick your phone.
You can also run apps from your SD card rather than having to transfer apps between the SD card and the internal memory whenever you want to use it. This is quite handy as you accumulate more and more shit on your phone.
The main reason that I rooted my EVO (aside from the fact that I'm a phone geek) is because I wanted to use my device as a wireless modem for my laptop when I'm traveling (wireless tethering). Sprint wants to charge me an additional $30/mo to do this. Fuck that. I already pay them for "unlimited" data, so I don't feel that I should have to pay even more money to use that data on my laptop vs. using it strictly on my phone. Obviously sprint feels differently, but fuck them.
The possibilities are literally endless. As Jordan mentioned, check xda-developers.net for more information. This is the holy grail of phone geeks for the Android OS. If you're trying to figure out how to do something on your droid, that's your number 1 go to place for information. I'd suggest going there and doing your research before you dive in to it. Read, read, read, read, read.