You heard what Jonny had to say about Castlevania: CotM. Now see the Luigi fan's take.
OK, before I start off, I need to say something. I am NOT a huge Castlevania fan. I have only played 2/3 of Super Castlevania IV (SC), and nothing else in the series. So, without further adieu, here is what I have to say.
The first thing I noticed in Castlevania, Circle of the Moon (CotM) was the sound. I am very impressed by this game's music. Like Jonny said, the opening theme is creepy, and seems perfect. It is a little metallic, as it has been downgraded to 8-bit for the output, but other than the slight high-pitch sound, it is nothing major. In fact, this "noise" will probably be found in any recorded sounds for every GBA game to come. You'll quickly learn to ignore it. The in-game music is very catchy, and sound effects seem to be very appropriately used.
The gameplay is very impressive so far. In SC, I found the whip/chain control to be very sloppy, especially when held down. Of course, in SC, you shook the whip around. This is not the case for CooM, thereby allowing me to play the game without a very soft pillow to punch. Jumping is also much less tedious in this game--although that may just be because the beginning doesn't start off with bottomless pits...
The graphics are the only complaint. As you probably already know, this game uses very dark tones to create atmosphere. I commend Konami for the idea, but it just makes the game harder to see. You may want to try using your light to help, but I'm not sure, as my light purchase didn't turn out well. The animation is, well, strange, but you get used to it after you play for a minute.
Overall, I am impressed with this game, and can fully understand why it is Famitsu's top pick for the GBA release. If I had a PSX, I'd go out and buy Symphony of the Night right now. But instead, I'll just get back to my new GBA game.