I had some GW flex head full sets years ago. Loved them, but can't say there has been a situation where the flex was 100% needed. The flex heads got real floppy on them and I sold them. Got along since then without any. This was back when the only store around that had them was Lowe's and still cost quite a bit so when somebody offered, I didn't think twice. Nowadays, I would think harder about it.
I currently have some stubby flex head SAE ratcheting wrenches. Again weren't 100% needed but definitely convenient. I think the stubby flex head is a bit more 'needed' than the standard length. Usually flex head ratchets help you get leverage with the dept of the socket being a factor. Ratcheting wrenches don't have as much of that issue, and you have the option of S/C/stubby to help out if they do.