Does Devil Fruit taste bad?
Yes, the Devil Fruit tastes bad. It tastes so inexplicably bad that is akin to poison as stated in the wiki about Devil Fruit. 

Luffy has been shown in multiple different ways eating his Fruit in the manga, anime and movies. And also how he got his Fruit is different depending on the source.

The main point is that, in almost every case, a person eats the whole Fruit either piece by piece or by swallowing it whole.