I agree that III is a bit easy, but only to those of us who have played it a ton. That's one thing I always liked about the Thunder Force series (well, 3/4/5, anyway) - they are memorization shooters, not twitch shooters. You get better every time you play through the level until you have it down - and then they are still fun because you get into the 'flow' when you play back through it.
And the music is to die for:
[youtube=425,350]MRDVRmbceSI[/youtube]
Can't beat that with a huge stick. I love it when the stage STOPS AND THEN STARTS SCROLLING BACKWARDS ><

I remember that part of their ad campaign was that the game featured 'line scrolling' - meaning they could change the direction that they are scrolling mid-stage as well as scrolling diagonally.
I for one am happy with it being at PS2 release. It will go great on my shelf next to Gradius V, the best shooter of last gen. It would be hella hot if they did PSN/XBLA releases of the previous ones tho. Or a PSP collection, I'd buy that immediately.