The shallow difficulty curve and lack of challenge in the main goals are the only things that let Soul Bubbles down. Its charm will keep you coming back, and it is well suited to short play sessions. That said, it's just too easy and doesn't really reward you during extended play time. The final quarter of the game does provide a decent challenge and is a worthy end, but it's entirely possible that you'll have drifted off by then or moved on to something that grabs and holds your attention a little earlier.
Soul Bubbles is an easy game to recommend to almost anyone despite its flaws, and in the end its charm wins through despite its shortcomings.