I've used CardStar since it came out.It's not perfect and some features (like trying to bring up your cards when you are at the store and open the app)
don't always work. Things overall are better than they were originally (they were acquired by constantcontact at some point, which may bother you as they
are an e-mail service company for businesses although I wouldn't call them a spam mill). I haven't been spammed by them as far as I can tell or their clients
(I use unique e-mails for different organizations so it is usually easier for me to tell where my e-mail got leaked if it does).
In the old days they did not require a data connection. They add a feature to do on-line backup, but I don't think you had to use it. But I haven't tried an exhaustive test and I do you the only backup feature (which is handy for transferring things to new devices now that it actually works as advertised?there have been bumps in the road along the way).
It will scan the code off the card, but it does not keep an actual image of the card. It figures out (at least in most cases) the barcode algorithm and card number itself.
It allows you to manually enter either.
With one exception (see below), most of the things that don't work are not the fault of the app: generally older bar code readers don't do well with the bar code displayed. I have not heard anything along the red/green lines. It is true the hand scanners do tend to be better. But not always. Overall, I think the situation here is improving as less ancient equipment trickles down to cheaper levels, but generally you will do better with "bigger' stores. Clerks are also not as surprised by it
any more even if they've not seen it and are more willing to give it a try.
CardStar does display the numeric code so if you/the clerk can figure out how to enter it, that can be helpful, but it isn't always obvious. For example, at one indy
market I liked (that alas has gone under), you had to enter the code as a UPC code (not a customer number) and not enter the check digit at the end.
At the Vons (Safeway) I sometimes go to, the clerks can scan your card with the hand scanner, but (like most places) the self-checkout doesn't have one of those
so you have to punch it in. Still you don't at least have to carry the physical card.
'Course, if your battery goes dead & you have no cards?
The once place I've find I have to carry the card is the local public library. Their scanners on the self-checkout can't handle phones, and, even if you go to the counter, they require a physical library card.
The one thing that is the app's fault, sort of, is Walgreen's, as they use an odd 2D encoding algorithm rather than one of the more common bar code algornithms,
so, although I can enter the number from the card, I can't generate the appropriate image. I think I tried to use a UPC bar code display of the number at Walgreen's and it didn't work, but I haven't tried anything else. I use passbook for Walgreen's and AAA (but I normally don't need to show an AAA card to anyone anyway if I use the app).