Don't know if you are talking US/EU but I usually queue myself in the Blizz other raids finder after work (Midnight UK) and have few issues, usually only accept invites who /w me first and are setting up a fresh run. I also rarely have problems with LFR but I don't often queue at highly popular times and whilst the queue is longer the player quality seems fine. The times I do have problems is weekend afternoon and any time I try to play during the late afternoon to early evening (Schools out hours). Best advice is to ask around on server or make a alt on a busy server and ask around.
Flex to me feels like LFR without the boss wipe buff, so people actually have to try and work together a bit.
In my experience, flex has been more of a hell than LFR. At least in LFR if people don't know fights, they will get carried by a few of the people that know what they're doing.
In flex, it's a similar calibre of player with less room for error. You're doomed to have a bad time!
Based on reading this thread it seems wow players just hate other wow players :P
It's terrible. 75% of the EU users are russians/spanish/french and it's sometimes impossible make a point. There's no easy way to check someone's experience and cyrillic letters + diacritical marks makes it difficult to find them on the armory. It's usually fine for flex 1 and 2 but I'll never use it for the latter ones again.