I can also confirm the simplicity, my wife use it all the time. It is simple to use.

It's a pain that it supports SMS. I wish they would make a TextSecure that completely dropped all SMS sending support. As it is, having legacy SMS makes it trickier to use than Whatsapp, since Textsecure "randomly" sends SMSs (e.g. when your data drops out). If SMS cost you money then this is a deal breaker for using it with less tech-savvy family members.

So obviously that's not the ideal solution, the ideal solution is to have an option to disable sending SMSs. Most people (at least in the US) have unlimited or nearly unlimited SMS with every plan, so sending an SMS doesn't really matter. For those that it does matter for, they could simply disable SMS. In fact, this is such an obvious solution that I went to check if it was there already and what do you know? It is. "Allow outgoing SMS to" has an a checkbox to allow outgoing SMS, to prompt before sending and SMS to another TextSecure user, etc.

I know that, and I can fiddle with settings and get it working as I like. Family members less so.

There are many data plans in Europe where SMS are charged at rip off rates, and are something to be avoided. Even just the risk of sending SMS is enough to put anyone off using the application. Especially when competing solutions work as expected - i.e. they will not send SMS at all, ever.

This is exactly why I don't use it.

Or you could just go to the "Allow outgoing SMS to" setting and uncheck the box that says "TextSecure users", or leave it checked and check the box which sais "Ask before sending SMS".