I used ROM Manager and tried a couple of the ROMs listed there. Most of them wouldn't even install.
The stock, rooted phone with the Sprint Apps locked works fine for my needs.
If I had had more spare time I might play around with different things. I also find reading the different threads confusing as they are all written for different skill levels and many are poorly written and hard to follow.
You did root before you tried to install a ROM right? ROM Manager is overused. Since you have it use it to install Clockwork recovery. Now go to XDA and download a ROM you want to try. Stick on the root of your SD card, boot into recovery(hold volume down while booting), flash it, clear cashe, and reboot. It's not difficult.
Do rooted phones have all the same functionality as a stock phone? In this case I'm thinking about the camera and 4G. I saw a bunch of rooted users complaining they didn't have these features.
Depends on the ROM. All have the camera with the exception of bleeding edge ROM's, currently the ones based on the desire. All ROM's that are based off Sprint releases will make use of 4G, currently pretty much all except CM and desire based ones.
Personally I use FreshRom with King's #11 Kernal.
This is something that requires some time on your part. You need to read, learn how things work, and the proper way to do it. It's not complicated or that time consuming. Spend a solid hour reading up and you'll be good to go. That is enough to learn how to root and install a ROM. If this is to much for you, then yes, just leave it stock.
#1 tip, use Titanium Backup. It will save you a ton of time reinstalling apps after flashing to a new ROM. If you find yourself doing it a lot upgrade to the premium version as it works faster.