LOL i jsut did this to my phone a week ago. evo 4g as well.
Do you have the newest update? i believe its 2.3.3 is the newest one. depending on your update will depend on what system you use to root it.
Rooting it wont change how your phone is now. it will work and act the same as it did before accept you can now download a wireless tether app so you can use your phone as a wireless router and use its internet for your computer. You can also download other apps that let you do more indebth things to the phone you couldnt before like removing all the apps your provider puts on the phone you cant get rid of.
Personally, once youget the rooting done, look at changing roms and playing around with kernels. its like hopping up your duramax but on a phone level lol. you can get better batt life, hav the phone work smoother and faster, and overclock the cpu for certain games and what not if you like. you can also add a sbc kernel that will charge the phone an extra 10% that HTC wont let happen for fear of charges overcharging the Li-on batt or shortening life span. its a long discussion on xda-developers.com but i run a sbc on mine and love it. battery last longer during the day and with the kernel im running i get better over all battery life.
its kinda risky if you dont follow the directions correctly but really im a total newb to that stuff and had it rooted in 4 hours (i was doing stupid things like not turing my anti-virus off which wouldnt let my phone work with the computer). its an easy 30 min now that i know how