The thing that bothers me, going by the PCMag data speed test, is that VM (granted on 3G) runs about 0.1 Mbps half the time all the time. Granted, 4G speeds are faster but 4G coverage is quite limited. The area I go shopping in, here in this major metro area that does have 4G coverage, I can't get 4G and my 3G is that 0.1 Mbps. I needed to use my Internet on Friday while I was running errands and things were ridiculously slow.
I'm strongly considering the $30 plan. I'm not a heavy phone user -- most of my Internet I do over Wi-Fi, so it doesn't cost me plan minutes. So, twice the bandwidth before throttling -- and with throttling not really being much worse than my current 3G speeds -- doesn't seem like it will have much of an effect. Also, if I get throttled and need the faster speeds, my understanding is that you can just start a new month early to get full speeds again.
I've used GrooveIP before and, again, since much of the time I am on Wi-Fi I'll set GrooveIP up for Wi-Fi only (unless I have a heavy usage month). I'll agree, GrooveIP can have issues but it integrates relatively well with the phone. So, in all, I see some real advantages in the T-Mobile plan.
Now, what does stop me is that I want my next phone to last at least 2 years. My problem is that I don't know how long T-Mobile will continue to honor the $30 rate because, I agree, the other rate plans are not appealing to me. And that is one thing I feel relatively safe with VM -- VM has never forced me to switch my plan when they change plans/rates. The worst is changing some details (such as the throttling) or requiring new plans for newer phones.