Simply the best side scrolling shooter on the C64. Period.
As you thrust deeper into enemy territory you are constantly thrown against waves of enemy fighters and larger opponents and the occasional obstacles such as walls and asteroid fields. You are awarded credits for destroying waves of fighters which can be used to purchase upgrades in the form of speed boost, increased firepower and multi-directional shots. The most expensive upgrades are ironically the least useful ones and don't come into play very often. An interesting twist is that the powerups will slowly start fading away, threatening to leave you defenseless at a crucial moment, which introduces some strategic elements into the gameplay.
One of the most appealing aspects of Delta is that it is truly a deep space environment with your ship floating in a dark starry space. The graphics are spartan but used to good effect, and have some neat visuals like your ship starting to glow red on the side facing a sun as you move closer to its corona. You will visit surreal environments from time to time, often with captivating names such as the Jelly of Dreams. While the sound effects are ok this game should be played in the music mode which offers an amazing fifteen minute long piece that adds an incredible atmosphere to the game.
There are no obvious shortcomings of this game, although sometimes a life lost at the wrong moment or the choosing of a wrong power up may force you to restart the game. Do not expect an easy ride, but if you give her time she will reward you immensely.