So I got my Ting phone on Saturday, and I'm happy with it. Phone-wise it's essentially the same phone I had before (Samsung Galaxy Nexus), only it's about 6 grams heavier and 0.5mm thicker. Oh no.
The voice service is pretty good so far, although I haven't used it much either.
The data service is passable. It's only 3G service for now, because LTE is only just beginning to be rolled out in Seattle (as a stealth beta), and I get around 800Kbps on a good day, compared to 10Mbps on T-Mobile. But that's enough for what I use data for on my phone most of the time, and in the meantime I have a Freedompop access point which I get 6Mbps on.
Unlike most pay-as-you-go/MVNO services, it actually lets you forward your voicemail to an alternate number, so Google Voice works with it with no voicemail conflicts (except that GV can't automatically provision it since it misdetects the number as being Sprint, but that's easily addressed from the Ting website).
So, in a week or two I'll just be canceling my T-Mobile plan and paying the $200 ETF (and reselling my GSM Galaxy Nexus). In the meantime, if you sign up for Ting via my referral link, both of us get a $25 credit, so that helps to lessen the blow. (And with my expected usage, that'll cover two months of service for me.)
One downside is that I really wish that in the lower usage tiers you could pay per unit instead of for the entire tier; I generally don't use text messages at all, and when I do it'll be 4-5 per month. At $3 for the first chunk of 100, that works out to 60 cents a text message for actual usage (compared to the 3 cents per message that it would nominally cost). Which I guess isn't too bad, really, but still, it's kind of offensive from a service-cost perspective. I suspect that most months I won't get any at all though, since I only use my phone's SMS gateway as a backup for my work pager and I hardly ever get paged. So, I expect my normal monthly bill to be $12-15. Which is totally fine compared to the $75/month I was paying before, for the same level of usage.