Is big Bend nat. park worth the drive 700+ miles ?

Definitely worth the drive from wherever you are.
You can ask lots of people what to see and do and you will get lots of answers. Big Bend National Park is something around a million acres and down the road is Big Bend State Park which is around 600,000 acres.
Chisos Mountain Basin is a good place to visit especially in the summer. It is high elevation and will be cooler.
The best time to visit is fall or spring but it gets pretty busy.
Terlingua is a good place to stay outside of the park.
It is amazing out there and there is so much to do. The first time I went, I was pulled over for speeding. I think the officer just wanted to talk to someone. He gave me some insight on the area and the biggest take away I got was he referred to things as "just right down the road about 60 miles". That sums up the Big Bend area for me. Lots to see and lots of driving.
The river road between Terlingua and Presidio is an awesome drive.
Lajitas has a cool area and nice golf course.
Don't be afraid to ride across the river and have lunch or drinks (assuming those places are still open) but make sure and take your passport.
I haven't seen it all but seen quite a bit out there. Hit me up if you have specific questions and I'll see if I can help.

All my life I had heard about Big Bend Nat'l Park. My wife and I were in Ft. Davis one year on July 4th, and we thought it would be neat to go down there, only about 100 miles away. We assumed there would be no rooms in the resort hotel, but we called. No problem, lots of rooms. So, we went. The park was virtually deserted. We drove all over, and only saw about ten vehicles. Boquillas Crossing was closed. Went to the resort and there were maybe ten rooms occupied. There were barely enough people to keep the restaurant open. We figured out that NO ONE GOES TO BIG BEND in July, because it's GODAWFUL HOT! Spring and Fall holidays, you can't get in.

The park is huge, and has miles and miles of nice roads, with numerous trails up into the scenic lookouts. We finally decided: There's Nothing There, But There's A Lot Of It!

Glad we went, but once was enough. IslandJim