I've stopped playing osu!std as of recent, but have continued playing osu!mania. osu! is a really awesome game, and provides a lot of gameplay and different maps/songs to play. It would take many years of playing to exhaust all of the different maps available, and many new ones are added every day.
As for the ranking system, it's a bit broken with some maps giving more reward than others for no real reason, but it's the best it gets, with occasional re-balances that make it slightly more fair. It can be a bit frustrating at times to make what you believe to be a really good play, but to get very little reward in the ranking system. It's confusing to understand what is a good play, and what isn't, but it really doesn't matter. At the end of the day, if you're playing just to enjoy the music and click circles, it doesn't matter at all.
Overall, osu! is quite a bit of fun, and even if you get bored of one gamemode, there's 3 others to try. There's almost always new songs to play, and new challenges to complete.