I previously didn't like it because it seemed to lack the tight level design of III and IV, and was choppy and generally ugly and glitchy looking. But it really does feel like Thunder Force. Although it's lacking the collidable scenery, the enemy patterns are all interesting and well-paced, and there's no shortage of neat ideas in the enemy designs. There's a really good variety here, from the tunnel with all the radars that send drones after you, the bonus level where you're in the Brigandine, to the immensely varied cyberspace level at the end.
It keeps the Cruise 'em Up feel intact, and there are many parts that I just enjoy playing every time even if they're fairly easy (like any of the parts where you're being chased by missiles, and taking them out with the free range

The free range really *is* overpowered, though. There's no better example of this than the st2 boss. Without free range, you have to learn several patterns. With it, the boss doesn't even get a chance to attack before she goes down. This is true of many sub-bosses, too. I don't totally mind, since you do need to learn the levels enough to hang onto the powerup. When I tried hard mode, I lost it pretty quickly. It's at least no worse than the Saber in TF3.
One question though: You get the bad ending if the last boss times out, right? And since it doesn't give you any powerups at the beginning of the level, does that mean that continuing here ensures that you can't get the good ending?