Despite severe slowdown issues, Gradius III is a competent and engaging shooter.
Although we didn't officially get Gradius II GOFER until way after the Super NES ended its lifespan, Gradius III was a fitting staple of the franchise. Almost all of the levels from the arcade made the cut, with the exception of the 3D bonus stages and rearrangements. To an extent, the graphics are close to arcade par and look fantastic given the hardware limitations. The music sounds even better than it does in the arcade with some truly memorable tunes. Exclusive to the console version are some new boss ships and a new level that literally has you at breakneck speed. Though such additions are few and far between, they're nice to have for the home port.
One of the biggest flaws of Gradius III, as mentioned earlier, has to be the debilitating slowdown issue. This is largely due in part because the Super NES didn't have the capability of processing multiple, heavily-detailed sprites at a comparable speed. So, in essence, stages like the Bubble level suffer enormously and everything slows to a crawl, rendering gameplay almost unplayable. The arcade game also had similar issues, but not nearly as bad as the home port. It can be argued that the game was trying to transition as much of the original source as possible, yet didn't realize how difficult it was to do so. Still, in all fairness, Gradius III was as ambitious in its development as it is being a moderately difficult and engaging shooter.
Gradius III isn't bad by any means--it's a great game that can provide hours of fun on your behalf. It's as close to the arcade version as it can get.