WolfBlade - Glad you're thinking of getting into this wonderful, delightful, colorful, enchanting, and thoroughly enjoyable game. Just a few more details:
The original concept was not as a "kiddy" game (I think the Chanel character dispelled that illusion pretty quickly) but as a date game. Specifically, a game that a boyfriend and girlfriend could play together (yes, it was originally intended to have multiple players). The colors were much brighter than in Beatmania, and it had cartoon characters with a variety of animations.
In the beginning, there was just Beginner (easy notecharts, limited songlist), Normal, and Party (that's the one with the "ojamas"). Since PM was not meant to be a murderous challenge-soaked nightmare, the majority of the songs were easy to clear. Hyper courses were introduced in 3. Once the game got popular with the really hardcore players, Konami added another level, EX, inaugurated in 5. But it wasn't until sometime later (10?) that Hyper and EX notecharts got scored separately. Before that they were all lumped into the 9 Key score.
The other major change was that the early games had 7 Key and 5 Key, essentially the 9 Key chart with the white keys and white and yellow keys, respectively, put on automatic, a la IIDX. Starting in 7, 5 Key became a completely separate difficulty level, and 7 Key was done away with.
There's a very good variety of songs. Much of it is gentle pop, as you might surmise from the title, but it goes into all kinds of genres, and surprisingly has more punk than I've seen in any other Bemani title. There are only a few really hardcore or blistering songs, and profanity is strictly prohibited (there's even one song that's full of bleeps). I wouldn't say the variety is a lot better than in IIDX, but it's definitely a much different flavor.
Some of the characters are really cute, some aren't so cute, and some are as adult and sultry and shameless as anything you'll see in Rumble Roses. For some reason, there's a whole bunch of new faces in each new game, and only a few get reused at all. A lot of them are based on real-life artists. Here are the most common ones:
Nyami: Cute, cuddly, sexy, and extraordinarily versatile anthromorphic yellow cat creature. Really came into her own starting from 6.
Mimi: Nyami's counterpart, exactly the same, except she's a rabbit. Or hare. I don't know which.
Rie: Mega-ultra cute brunette 18 year old who looks a lot younger. Variety of callings, including cafe waitress.
Sanae: Very similar to Rie, but with black hair (dark brown in some games). She's primarilty a singer and also plays a mean acoustic guitar.
Mary: Way-hot fairly dark-skinned professional dancer with an afro. I'm pretty sure she's supposed to be black, but Japan has a problem with very dark skin for some reason (cf. Elena from Street Fighter 3). Japanese.
Judy: Incredibly sexy California-born blue eyed blonde professional dancer. Mary's soul sister.
MZD: PM's never-aging DJ. Has nearly all the "Remix" songs.
Hamanov: Chicken orchestra conductor; master of the Classic songs.
The King, Aya, and Pretty also show up pretty frequently.
As for timing windows...they vary a lot, much like in the original Beatmania. You definitely won't be able to 100K
everything in 9 Key, although if you play long enough you should manage at least a few. For the most part, the challenge isn't the timing, it's keeping track of nine buttons and hitting the right one every time. (This absolutely
killed me when I was first starting out.) So long as you don't shoot too high too fast, you should be able to find something you're comfortable with.
Okay? Okay. There's tons more to tell, but this should do until you actually start playing. Which you definitely should.
what exactly is it? 
