Comparisons to Castlevania, and in particular Super Metroid, are spot on. It's a real crate and grate sidescroller, with plenty of optional exploration, backtracking, and stealth. Most importantly, Chair's nailed all the basics--shooting enemies in the 2.5D environments is immediately satisfying.
The story may be forgettable, and sometimes shooting guys in the background is tricky, and the bosses aren't perfect--but beyond that, it's tough to criticize. In this great season of downloadable games, it sits at the top with Battlefield 1943 and Splosion Man. The game hits Wednesday for 1200 MS points ($15). Highly recommended. BOOM video 2403