"It is pretty delicious."
けっこう implies not perfect. So you should not use けっこう in all situations . just おいしい or かなり おいしい are prefered. we use it as follows.
it is cheap but けっこう おいしい. ... it implies better than I expect and also implies the expectation is not so high.
it is expensive so かなり おいしい.
けっこう (結構) has a couple of meanings. Here, where it is used as an adverb to modify an adjective, it gives a kind of limited endorsement, as pointed out by nakada, and could be translated as 'reasonably,' 'fairly,' or 'tolerably' (tasty / good ... whatever).
However, as an adjectival noun (= words that must add な in the attributive pre-noun position), it is a much more solid endorsement: 'splendid,' 'wonderful,' 'fine,' 'delicious.' There is a well known saying: Don't say けっこう (splendid) unless you have seen Nikko (日光), the location of the mausoleum of Tokugawa Ieyasu, the founder of the Tokugawa shogunate. In Japanese, the saying is 日光を見ずして結構というなかれ.
There is at least one other common use. When offered food, one can say (もう) けっこうです 'I'm fine (already)' in the sense of 'No, thanks,' though sometimes people seem not to be sure whether you meant to politely refuse, or whether on the contrary you meant (yes) 'that would be fine.'
You might be able to clarify the 'No, thanks' negative intent of けっこうです with some body language: while slightly inclining your head in polite acknowledgement of the offer, you simultaneously raise your flattened right hand in front of your chest and neck, palm facing left & little finger edge outwards, perhaps shaking your hand very slightly. You'll also see this hand (and head) movement with しつれいします, 'excuse me,' or 'I'm afraid I have to leave.'
おいしい is an adjective, not a noun. けっこう must therefore be an adverb, rather than the pre-noun attributive form of a な-adjective.