I recently--OK, yesterday--switched my Nokia Lumia 810 running Windows Phone 8.0 from T-Mobile to Cricket Wireless. After getting and entering the SIM Unlock code from T-Mobile, I ordered a BYOD Micro SIM from Cricket.
I activated the Cricket SIM and placed it into my phone: after a couple of minutes, everything mostly worked.
I was unable to send or receive MMS texts, however. I chatted with support at Cricket but didn't get anywhere. A brief search, though, turned up the Access Point app from Microsoft Mobile. After downloading the app, I found that AT&T--the network that Cricket uses--was active. I changed that to AIO Wireless, stopped and restarted the phone, and everything now seems to work. An added benefit seems to be that Internet Sharing is now enabled (something that you had to pay extra for with T-Mobile).
The phone indicates that it connects to the 4G network and I'm getting 4 Mbps/1 Mbps down/up speeds which is nothing to write home about, but that's about as good as I ever got from T-Mobile.