Painfully hard. Amazingly addictive.
Gradius has a simple upgrade system - you can choose from a speed powerup, double shot, missiles, laser, an extra little ship that shoots things, and a shield. This makes the game have some strategy.
The game is incredibly hard. There are tonnes of enemies which often shoot many bullets at once, there are environment based obstacles (walls in your way) and you won't get through the first level without a lot of practice!
This adds to the achievement of beating a level. At the end of a level there are challenging bosses which get harder and harder.
Graphics are pretty good for the NES but not the games selling point. The music and sound is good - effects are alright, though the music is either love it or hate it - it can be really annoying.
Gradius has no save point but it does have a handy cheat code which helps a lot when you are in a later level with no powerups!
Gradius is one of those games you'll just keep playing. It will take some time to beat.
Gradius is available on Wii for 500 Wii Points.