All I can say that it is a very commendable game that pays tribute to Gradius with a few little unique points, such as options being required to upgrade twice for later unlock-able types compared to easy upgradable missiles and main shots for the first three types. Additionally, it's great that each type has a special button to play around with Option formation, which is expanded from Gradius V. While I do wish those games that involve the upgrade gauge can be experimented further, at least this game fills a good niche for those wanting to crave some memories of playing Gradius games.
Not to mention that the three level routes distinguish yet delightly derive from the first three Gradius games in term of theme. Plenty of hard stuff to get people replaying this continuously, and to grind for more points to unlock more types.
The only concern is the amount of points required to spend on the unlocking types that take longer than desired. Maybe shortening the amount of points required could allow more flexibility with various types sooner.