I love the insider hookup and the service (particularly in big cities like Seattle),  but be careful when you add a new line, we just did and there are some peculiarities with doing this:

1. Even if you have Autopay enabled, it will not give you the Autopay discount unless you call. This is apparently because their computer system is that it doesn't notice your Autopay discount (it's significant!) for a few months.
2. Your bill will appear to spike. The reason for this is that say, you in the worst case, buy a new line one second after the start of the month. It will not pick up the new line until the month after the next. The reason is that apparently, the billing system uses the number of lines open at the start of the month for it's count. It then bill at the end of the month. In some sense, you get a "free" month with your line, but then it catches up the month after that. And yes, I had to talk with the T-mobile rep for a while before I got this.

Final tips,

1. Wait for sales, they are pretty frequent, for instance, we got 4 lines for $160 with Autopay. 2. And it sure helps to have a friend at T-Mobile as well as they have a nice program that provide an additional discount that you can stack. 3. They also have promotions for the T-mobile SIM card as they charge$25 for activation. You can also typically get a prepaid T-mobile SIM and if it is less than \$25, then you can use that instead. Also many times during the year, they don't charge for activation.
4. If you are a frequent Alaska Airlines flyer, they give you one hour free access on the plane. And if you are lucky, they will even throw in T-Mobile One Plus which gives you unlimited access. Note that if you are on a family plan, you get one hour per line so with five lines.
5. Last note is their Canada and Mexico roaming works well and the 2G access internationally works well in Europe for sure, it is slower for images, but perfect for iMessage, texting and email.