Got to stick up for one of my favorite Castlevania's here. While I completely agree that Aria is easily the best of the GBA trilogy, and IMHO of all the Castleroids, I never understood why Circle gets so much more love than Harmony. Let me hit you with some knowledge:
First off, you DO NOT have to go into a menu to switch between spell books; try holding up, then tapping L or R to cycle through your available books. You also DO NOT have to go into a menu to switch between your spell book or weapon; try holding down while tapping L and R together. I always found these shortcuts extremely convenient, elegant, and a boon to the overall flow of the game.
Second, you say the game is overly easy? Try a naked run (i.e. don't equip any boots, armor, etc.), where even early enemies will OHKO you. Combine the spell book system shortcuts with a naked run and you will realize the beauty of Harmony: you don't HAVE to go into any menus at all (well, except to equip a select few items to progress, of course). Coupled with dedicated forward and back dash buttons, Iga created an extremely fast, nimble and exhilarating gameplay experience which can only be found in this entry.
Finally, I don't understand how you can call Harmony an "ugly" game when compared to Circle. I'll take the overly bright colors found in Harmony over the horribly bland and muted tones of Circle any day. Remember that Circle and Harmony were released on the original GBA, which did not have back-lighting. The blue "mouse trail", larger sprites and brighter colors in Harmony were implemented to make up for the hardware's deficiencies that made Circle unplayable, except under ideal lighting conditions. Also, I must disagree on the multi-segmented enemies, as they look amazing, and add some welcomed flavor and awe to the game.
In the end, I do agree that the castle in Harmony can be confusing at first, and that the music is definitely nowhere near as good as the tunes in Aria or Circle. However, its the GAMEPLAY that really matters boys and girls, in which regard Harmony shines, and should not be missed.