An alternative take from contemporary viewponts on this game.
To put it simply, the gameplay involves numbers and maths, into both comes a bit of number logic. So, yeah. It's not hard, especially since most of the problems can be solved via a guidebook you're given in the game. If it wasn't enough, you'll get quite a few hints while playing, which would explain the lack of walkthroughs.
However, I believe that the main aspect of this game isn't the gameplay (can't believe I said that), but it's the story and the character interactions. If you've played Killer7 or No More Heroes and you like how random events link together to form a relatively incoherent story open to interpretation (more Killer7 than NMH I guess), then you'll like the story in Flower, Rain and Sun.
The characters in the game are again as random as expected from SUDA 51. You'll love the way Sumio Mondo (the protagonist) interacts with those around, more often than not being a tsukkomi (roughly translated as a straight man in a comedic duo) to the game's gags.
Yes, I know, the main premise is stopping a terrorist bomb. That doesn't mean the game is dark and gloomy, rather it makes fun of itself and have more of a light mood to it. Some people may view this as an ego-trip for the developers, having fun at the expense of the players. However, I believe not having a serious mood actually makes the game more playable than it should be (especially with the lack of intensive or interactive gaming).
I guess what I'm trying to say is that, this game won't be everyone's slice of cake (hmm... better make that the majority of people's cake then). However, this game has in it a great story you don't see too often now-a-days. In summary, the game's a blast to play for its story and characters, however if you're looking for more intensive/immersive gameplay don't expect much beyond solving hard number puzzles.
Story: 10
Gamplay: 5
Total: 8
Don't bother asking me for graphics or sound scores. I play any game I like regardless of either. That said, the graphics in this game is decent for a DS and the music incorporates some of the more recognisable tunes from the classical genre (which is really nice).
edit: Partially testing the edit function onthe review section, I would like to add that one thing that may put a lot of people off would be the amount of walkin Mondo has to go through. All I can say is persevere. The story's worth it. ^ ^
~nya