I have been using the Shure SE215 earphones for two months now while riding EUC, and I am really happy with them. Bought them used. I am quiet picky when it comes to sound, but for the price of these I am very impressed! They also have great passive sound isolation. I don't know exactly how much they block out, but I would guess somewhere around 25 db. It is very noticeable, and it pretty much blocks out all the wind noise, going PEV speed. The way the wire goes above your ear, makes it easier to put on a full-face helmet, than it is with most earphones. Since they have wire, you don't need to worry about them falling out, when you take off your helmet. They are comfortable to use, even with a full-face helmet.
I have only used them while riding, so I can't comment on how good they are for everyday use.
If you are looking for a good set of earphones to use under your helmet, you can't go wrong with SE215. I highly recommend them!