I agree that a lot of the reception issues may not be T-Mobile's fault but rather the abysmal antenna in the Sidekick line. I had both a Sidekick 2 and a 3 and was amazed at how much better service I received when I switched to my 8800. I now have the 8320 because when I have poor service, I'm usually in an area with Wi-fi that I can connect to, which more or less doubles my coverage area.
Can you try the Blackberry for a 30 day trial? I think T-Mobile allows this. A data-only plan for Blackberry runs 39.99 a month on T-Mobile, as seen here:
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