first time i used chest i got legendary level 9 pistol. used it till about level 25. used about 20 keys since- never saw another legendary....but it does in fact give them- but about as often as the vault symbols on the slot machine.
Save them for the level cap increase. I have 27 now and am going to use them when I've reached the new cap so I have at least decent weapons to use while beginning farming. My group has over 100 now so hopefully we get SOMETHING useful out of them because we didn't after using like 70 when we reached 50 and that was quite disappointing. Of course we already had the CC/Bee combo then so I'm sure our standards may have been a little high lol.
i know, i read that too--but i got one- first time i ever opened the chest, first night the game was out. first thing i did when i got to sanctuary. i kept it too- when i get home ill tell you what it is. your also not supposed to have unlimited bank slots, or infinite ammo, and i got that too... this game is so whacky!