These are very nice to try T-Mobile MVNO service. You get 2 SIMs, one intended as a trial, and the second to start a plan. Both can be used as trials if you want to, or give one to a friend to try.
Both include 100minutes, 100 text, 100mb and are good for 7 days of use. They are activated through a special app. (I've you're thinking of keeping one for a possible emergency, pre-load the app on your phone via WiFi.) You may need to use a spare email address if you've tried Mint before, etc. There is an expiration date listed on the trial kit (a few months, IIRC)-- I activated one successfully several months after the listed expiration, FWIW.
A note about the Ultra trial: Ultra's recent promotions have had some odd exclusions for phones that had been used on T-Mobile service before, so. this may be a concern. (I imagine that the Ultra trial would not cause problems, as long as one stayed well within the fences of the Ultra offer.)
You cannot use these trial offers to get a number to port elsewhere-- they won't provide you with account number and PIN needed for porting out.