Agree to get the Flex if you're in VA. The ONE time you can use it on the express lanes it's well worth it!

I drive 95 and 495 express every day with carpoolers and it's life changing to not have to sit in traffic or pay $60 one-way to get to work.

OP here: We don't have one because we mostly commute between Alexandria and the city. We're only on one of the toll roads every couple of months. And when we take the beltway, it's usually on a weekend and there's no traffic, and even that's pretty rare. So we only have a need every few months, but every time we are stuck paying a toll (or get stuck on the Beltway), I kick myself for not getting them.
So thank you for the advice--it's really helpful!!

We routinely use EZ Pass to visit relatives in New York. Works in every state we go through.

We have never had any problems using it. We have several for all cars in family and all on the same account.
I haven't had any problem in DC HOV lanes either.