You'll fly through it due to its lack of difficulty, but those handful of hours are certainly fun and unique.
As was prior mentioned, Kirby's ability to inhale an enemy and copy its abilities is what keeps the game so fun. He can inhale all but bosses when he has no copied ability, which alone is rather powerful. He can suck in any enemy from within a few character lengths directly in front of him, essentially leading to an instant kill. He can then assign that enemy's special power to him, granting him a couple dozen possible skills. He can turn into a rock and crush whatever is beneath him, fire lasers, electrify himself, rush forward as a fireball, turn into a tornado, swing a sword, float on an umbrella, freeze enemies, and so much more. Getting hit will cause him to lose that skill, but he has a few seconds to inhale a star that bounces out of him to regain the skill. With the ability to absorb six hits before dying plus his offensive arsenal, Kirby will have little trouble with anything outside of boss encounters.
Another of Kirby's unique traits that throws a wrench into the traditional platformer formula is his ability to fly. By hitting up, Kirby inflates and can float about indefinitely, and can deflate and fire a burst of air at an enemy as a weapon. Though he is understandably not very agile while airborne, it all but takes bottomless pits out of the picture. In fact, in a few levels it is quite possible for Kirby to float over just about all of the dangers on the ground, which certainly allows you to easily and quickly pass some segments of the game.
The boss fights are really the only segment of the game that will prove in any way challenging. For a NES game, the bosses show a good deal of creativity, with certain tactics having to be developed on the fly in order to succeed. Living trees, the sun and the moon, and even a mechanical drill burrowing through the earth all await you, with a few attempts likely necessary to take them down. In the end, though, they're the only speedbumps on your journey through the game. There are 42 levels in all, plus the main boss battle at the end of each of the themed worlds. Some levels contain an alternate exit which leads to things like a simple minigame that can reward you with extra points or lives, but there are no actual hidden worlds or really any way to skip them. A playthrough with the intent to simply pass the levels and beat the game will last a solid evening of gaming at best, and even searching for the exits does not provide much more. Without much replay value, it should be noted that this $5 investment will likely not last you an entire weekend. Still, the content there is good and it stands up favorably against other titles from its console.
As far as NES games go, Kirby's Adventure is among the best you'll find in terms of graphics and audio. The sprites are relatively detailed and show a good range of movements, and a number of them can move about on the screen at once. The themed worlds have a nice flair about them, and show a good range of detail. Some levels even have some impressive rotation effects that can give a good sense of scale. However, a few areas can slow the game down to a crawl, with the Spark ability being the worst offender. The music and effects are of good quality given the sound synthesizer that the NES has, and there are some catchy tunes found throughout.
Kirby's Adventure certainly has its charm and can be rather fun to play, but a relative easy ride means that the whole game will last only a handful of hours. Among the NES offerings on the Virtual Console, it's among the best games that do not feature the names "Mario" or "Zelda" in the title. It brings something new and unique to the platformer genre, but a couple of design decisions prevents more than a half dozen hours of play at best as there is not much reason to play through more than once. If you're looking for the most bang for your buck there's better games available both on the Virtual Console and in the used game sections, but if your collection is of a decent size and are looking for something new for a rainy weekend, then Kirby's Adventure is worth a try.