Thanks for the kind welcomes! Just to be clear, what I've got is an SGS2, not an SGS3.
I could have gotten the SGS3 for about $200, but I figured these old S2s give you 90% of the S3's goodness, and I didn't really need that remaining 10% enough to justify spending $200.
In answer to Trooper's question, I took my browser to Speedtest.net, got automatically redirected to Google Play, and installed the app.
Sitting here in my home, and connected to a server in Palo Alto via the 4G network, here are the results Speedtest generated:
Ping: 124 ms
Download: 203 kbps
Upload: 838 kbps
Regarding T-Mobile's service, their coverage areas are good. What I really like about them is that, unlike all the other major carriers, there's no smartphone surcharge. That is to say, you're paying for the same data plan, at the same price, regardless of whether you have a feature phone or a smart phone. I really like not getting penalized for having a smart phone.
Say, is there something I should be doing to make my profile pic show up next to my posts? I went looking for a nice image that would be in keeping with the theme of androids and I found a good one.