I have the Samsung Epic 4g Gingerbread EI22 2.3.5 and recently rooted it. I tried several different custom ROMs for it and semi-bricked it a few times. Like mShred said.....this can be undone, as long as you can get into download mode. Using the program he mentioned (Odin), you just flash the stock firmware back to the phone. XDA developers is a great forum for all things cellular and there are a lot of videos on youtube. QBKing77 has a lot of rooting videos on youtube and he includes file links in his descriptions.
I personally didn't care so much for the "one click root" method, but that's just me. Here's a basic run down of rooting my phone:
The build number of Gingerbread.EI22 (Settings->About Phone) needs a GBkernel file to properly be rooted so I downloaded that file to my desktop. Also downloaded the ACSRecovery file to my desktop. I downloaded the superuser file to the sda card on my phone. Power off the phone, put it in download mode. Open Odin and add GBkernel into the 'PDA' slot. Only have 'reboot' clicked. Plug usb into phone & computer. Odin recognizes phone, hit 'start'. Receive 'Pass' message and phone reboots.
Power phone off. Put back into download mode and this time use the ACSRecovery file in the 'PDA' slot. Let run, reboots. Power phone off and put into 'recovery mode'. Using the ACS Recovery software, install the superuser zip file from the sda card. Reboot phone. Phone should be rooted.
If I didn't have the EI22 version, I could've skipped the first part and just used Odin for the ACSRecovery file, then in recovery mode installed the zip file from the sda card. Simple. Then I downloaded 'Rom Manager' from 'Market' and got the ClockworkMod file. Got Titanium Backup (whew!). I backed everything up to my sda card using Titanium Backup and then played around with different custom ROMs. You can download the custom ROMs to your sda card, use Rom Manager to install them or power your phone off and then in recovery mode, install it. The first custom ROM I tried wouldn't let my phone get passed the 'Samsung' screen on boot up, even after reinstalling the ROM, so....downloaded stock firmware to desktop, pulled the battery, put phone in download mode, used Odin to flash back to stock....then rerooted. Tried a second custom ROM....success! Well, I thought so, but kept having a lot of glitches with it until it went haywire so I tried a different ROM...success! Well.....after every possibly 'force close' message, I found that if you go into 'Rom Manager' and fix the permissions, it helps. After a reboot though, I was now getting a message that my internal storage space was full and the phone kept stalling.
Back to stock flash. Rerooted. Tried another ROM. Success! but....didn't care for it
Finally, went back to stock, rerooted, and stayed there. Phones been functioning just fine since. I used Titanium Backup and reinstalled everything from the sda card (life saver). Deleted stock apps and with root I can use my phone as a wifi hotspot without the extra monthly fees.
ROMs I tried (not in any order):
I'd suggest watching several videos the whole way through and reading some tutorials first but then...go for it!