I've had the Zio for three days now. I walked right into a corporate Cricket store here in Cincinnati and they had some in stock - the rep told me they initially got four, and then got eight earlier in the day. So supplies right now are limited.
Overall I like the phone, but do have a couple of minor quibbles:
The screen isn't as touch-responsive as I'd like it to be, compared to an iPod Touch.
Battery life isn't that great.
I don't like how the keyboard pops up when you go to read a text. Haven't found a way to turn that off.
Speaking of the keyboard, it's way too crammed in portrait mode. If you've got wide fingers (I don't...), it'll be a nightmare typing.
This is the first time I have owned a "smartphone", and overall it's not a bad phone, given its limitations.