I've heard about osu! for a while now. While I love music fervently, my heart was already taken by the Project Diva series of rhythm games. Unlike many other rhythm games, which mainly focus on throwing out notes on the beats at the song's tempo, and adding more and more notes on half-beats, quarter-beats, &c. as the difficulty rises, Project Diva's pattern actually match the song! That might not sound like such an achievement, but you'd be surprised. Not only that, but with its unique engine, where each note has its own hit zone, Project Diva's engine allows it to make unique patterns which utilize the entire screen space and uses positioning and the notes to relate to the song, a fact which is taken advantage of by both its developers and fans who have made their own songs for it. (But I'm getting off track here.)
I would like to start off by saying that osu! is a good rhythm game (although, if you don't like rhythm games...). By abandoning the antiquated "single hit zone with notes floating generically across the screen toward them" model, osu! really takes advantage of the music and tying its beatmaps to the songs. Remember, the only difference between rhythm games and Simon Says is the music, so if the game doesn't properly integrate the gameplay with the music, then it's no good!
That said, osu! is definitely not perfect, and it has many flaws, and it saddens me that these are mostly easy to rectify. First of all, if there's one thing that its developers could do to greatly improve the game, it would be to remove the spinner. Project Diva had only a single type of note, the note, and Project Diva 2nd only added two types of notes, the held note and the double note, but because each note could be placed anywhere and come from any direction on the screen, those simple notes could be used in as many ways as there are songs. osu! has the same advantages with its regular notes and its sliders, which really immerse you into the game's rhythmic clicking, and then, all of a sudden, SPINNER! QUICK, SPIN THE MOUSE LIKE CRAZY! OH WAIT, BACK TO REGULAR NOTES NOW. Like that previous sentence, spinners completely destroy the immersion of the gameplay. Furthermore, custom beatmaps tend to modify the spinner with a clear wheel, which doesn't help at all.
The second easily fixable flaw, although it is much less severe than the spinner, is the multi-repeating sliders. In addition to regular sliders (osu!'s version of the held note), there are also slider's with a repeat sign at the end, signalling the player to go back along them; this is fine. However, in addition to those are multi-repeating sliders, where after the player gets to the end, a repeat sign then appears at the beginning, telling the player to go back again, (and again, and again, depending on how many repeats the slider has), which, unlike the regular repeat sliders, where the player gets a full period to process that information, the player only knows there's an additional repeat with either prior knowledge (which is bad game design if the game relies on it), or the half second between reaching the end of the slider the first time and having to repeat again. The player doesn't even know how many repeats there are without playing through the beatmap once and remembering it. This is partially allieviated by longer sliders at moderate tempos, as the player has time to react, and if the player knows the song well, as the sliders tend to match the song, but with short, fast sliders on songs the player may not know well, having a slider keep on repeating when the player thought it was finished and has moved on to the next note is disconcerting and breaks immersion.
Other than these two (the first of which is extremely annoying, and the second of which is only mildly annoying), osu!'s flaws are minor. You have to download the beatmaps separately through a web browser, which could have been done through the game client instead to streamline the experience. There are also some other flaws, which are so minor I have forgotten them at the moment.
Despite my ranting criticism above, osu! really is a pretty good game. With the user-generated content aspect (in fact, most, of not all, of its beatmaps are user-made) like Project Diva or LittleBigPlanet and its multiple-possibility-simple-mechanic design ideology (i.e., game mechanics comprise of simple notes and simple rules, but they can be implemented in an astounding variety of ways), osu! is definitely a game to try if you're in to music, rhythm games, or game design. For the final group, notice the flaws i pointed out as well, so that you may avoid them in your own games.