I'm not even sure you're going to be able to find that plan anymore. [EDIT: dangrossman's right, they still have this plan on the "prepaid" side, but you have to dig a bit. If you're a brand new customer to T-Mobile, you're not exactly going to stumble on it from the main page.] The website really looks like they're bumping everyone up to the $50+ plans, which is sad, because they owned the value data plan territory for a while.
Case in point, I'm currently paying them $25/mo for 1.5 GB of high speed data. No contract, BYOD. I originally thought I needed "unlimited" data, but found that if I just avoid videos until I'm around wifi (which I'm around way more often than I thought I would be), then I never touch the limit.
So how's the $50 per month for 500 MB high speed plus unlimited everything else compare?
Ting offers 500 MB for under $20 / mo, and you can even pay-as-you-go to save money, or cover overages at a reasonable rate. For $50 per month, Cricket gives you 2.5GB of 4G speeds per month, falling back to unlimited 3G speeds, 5GB for $60 or 10GB for $70. Neither are BYOD, though, which can be a deal killer.
T-Mobile had great pricing, but now they're just normalizing with the larger players like Sprint and AT&T, drifting away from the bargains offered by the mobile virtual network operators (MVNOs), probably pulling the whole market upwards.