I tried the Spanx once, and I couldn't wear it. I bought the one you have pictured and it kept rolling down at the top. So I ended up giving it to my sister. They make little clippy things that attach it to your bra, but the dress I was wearing
was a thin cotton jersey, so the clips would have show. I've worn a long line strapless bra before and, I agree, it was uncomfortable. But I also had a really cheap one. Perhaps try a few more on, maybe trying out different sizes to find one that is
more comfortable. Fredericks has a few.
For my wedding, the seams of my dress kept the girls in place with the help of a little double-sided tape. I had to go commando though. The dress showed every kind of panty line imaginable. I did get a little tired of sucking my stomach in
towards the end of the night. DH kinda liked knowing that I had nothing on under my gown. ;-)
If you can go without the undergarments, I'd say go for it. Keep that light and comfortable feeling. ;-)