If you intend to use it with something else it's fine, otherwise it will get your gag reflex going after a few days.
I use unflavoured in a Nutribullet with a banana and oats in the morning, but flavoured (usually banana, strawberry or chocolate) for post workout or when used alone.
If you've bought it there's loads you can do to help the taste, get another flavoured bag and mix the two in your shaker? Alternatively, add honey, Nesquik, blend with bananas or other fruits or even use it in your cereal.
Although, my aim isn't to get lean. I'm a Power lifter, so i eat anything and everything.
p.s I would never buy MyProtein Casein. The texture and flavour are horrible and the amount of froth it produces with water make for a lot of waste (I know froth is a sign of a more quality protein, but one doesn't have time to wait for froth to settle) - opt for Optimum Nutrition Casein Instead!