C6606 user here. Currently running @niaboc79
4.3 Black Edition, and couldn't be happier.
I got my Z shortly after it became available, and have had a custom stock based ROM on it since day 1. I will admit there was a slight learning curve, but I knew I was switching to T-mobile and started reading up on the whole rooting/flashtool before I purchased and have stayed out of trouble (for the most part). Once you flash a ROM or two you will get the gist of it and it will become less confusing. The beauty of it is that we don't have to wait for T-Mobile to push out new Android versions. Once the Z gets it, so can we and no T-Mobile bloat. You will not have wifi calling if you haven't figured that out already.
Look for the "LB" (lockedbootloader) tag in the ROM title, and they should be good to go for us C6606 users.
Just follow the OP's instructions for installing and you will be fine. I haven't touched anything directly related to the C6606, other than APN's. I haven't had any luck doing the "download settings for internet and MMS" under wireless & networks settings.
I use these APN's every time I do a clean install and all network modes (3G, 4G, LTE) work just fine - http://forum.xda-developers.com/show....php?t=2236735