I have a pearl, its badass! by far the coolest phone I've ever had. I had Nextel when it was just nextel, and Nextel when it was Sprint. They both use SIM cards as well. Nextel on its on had awesome coverage/service, when they joined Sprint it went to the crapper, and the text messaging was never good.
My Pearl coverage with Tmob isn't bad. Not as good as nextel, but ok. You can change the OS on the phone to any that is available, even those from other carriers. Different OS's yield different things, faster phone, quicker reads of sd card, less dropped calls, different options, etc.
You can use the Pearl with any carrier, just unlock it.
Check out some of these educational sites/forums:
www.pinstack.com
www.eveek.com
www.phonescoop.com