Project Fi canceled my Google Voice account during sign up, but it transferred all my GV account credit over to the new account. The credit wasn't visible on the first bill but appeared on the second. It was nice not to lose those GV funds. (Fi also transferred my voicemail greetings and blocked numbers from GV).
I really like the way that Fi charges for data. I chose the 1 GB plan (since there's no penalty for overages). Last month I consumed 1.165 GB of data. On Ting I would have paid $10 for crossing into the second GB. On Fi, I paid only $1.65 extra; exactly covering my overage.
Fi provides free data-only SIM cards whose usage is just added to your account. Since my family uses VoIP for all phone calls (either SIP or Google Voice) and SMS, everyone just needs data. I gave everyone a data-only SIM card and it's been working great. The SIM cards work in every device we've tried, old and new. This avoids the $20/month charge per phone line. Fi billing breaks out usage for each device. The only downside is that you can only order one data SIM at a time. It took me a few weeks to place orders for all the cards I needed.
Ting is built on T-Mobile's network. It has great coverage almost everywhere I go. The one exception is the northwest quarter of my grocery store. Data signal in that part of the store was always missing.
Since Project Fi automatically switches between T-Mobile, Sprint and US Cellular networks, depending on signal strength, I expected Fi to do better. It did. This dead spot is no longer a problem. In this particular store, Fi often switches to the Sprint network then switches back to T-Mobile when I go elsewhere. Never underestimate the power of a layer of abstraction. Data-only SIM cards only use T-Mobile, so their coverage is identical to what I had on Ting.
I use Signal Spy to see which network my phone is currently using. I find it gratifying to drive through town and watch Fi switch networks. I look forward to some road trips this summer to see how effective it is in that scenario.
Fi's switching algorithm sometimes sticks with a network whose signal is slightly weaker than the alternatives. Theoretically this could hurt battery life, but I've never had it impact connectivity. It does annoy my OCD a little.
Overall, I'm very happy with Project Fi and will probably stick with it after I return from Europe. I hope Fi's international coverage is as good as the US coverage.