After cooking, does it just work?

[Geekcop]

Welcome to Geekdom! The first thing I tell anyone new to the Geek game is to not be in a hurry to flash a new ROM. There's a couple of reasons for this. First, you need to take time to get to know your device and how it performs under the stock ROM. This way, you'll better appreciate the changes that the custom ROM brings to the table. Next, make sure you completely understand the flashing process. This is where things have a tendency to go terribly wrong if you don't follow the instructions to the letter. One minute you have a cool PPC, the next, a cool paperweight.

To answer your questions, you will not have to re enter your ESN, but you will have to reactivate your phone on the network. Also, you phone will likely act differently, depending on the ROM you select. Network communication shouldn't be affected, but again, when you flash a custom ROM, anything is possible.