For the record, you can choose Classic style from the get-go. Choosing it is the correct answer.
PPG: DOT is a 2D Metroidvania that has you flying around levels rather than running (comparable to Aquaria), with some shoot'em up elements (namely, a bunch of projectiles flying at you). The plot is that Mojo Jojo has erased the memories of the girls' superpowers (but not each other, oddly enough). As a result, you collect memories (which enable you to use your powers again), starting with the basic punch. Why do you need to remember how to punch something? Speaking of, which can't you jump before you can fly? It goes from the basic Powerpuff powers like flight and laser beam eyes to abilities that can help you in combat, like the ability to cancel out energy attacks by punching them. Your powers use up, well, Power, which has a Power Bar, along with your Health Bar. Both of these can be upgraded. You can also unlock Boosters, which give you further benefits, but you can equip a maximum of 3 out of 5.
The graphics look like a browser game, though the girls look on-model with the show. There are cutscenes that are voiced by the original actors/actresses, but the characters are static in them. Would've been nice to have them fully animated. There are a variety of robots to fight (though most are just round, blue things that shoot at you). There are 4 bosses including Mojo Jojo himself, so unlike other Metroidvanias, fighting a boss is not a precursor for gaining a power, though the first two will unlock Blossom and Bubbles (you start as Buttercup). Each girl plays largely the same, though each has their own special power. You can switch between them on the fly by pressing LB and RB (on the 360 controller anyway). Buttercup's Shockwave can go through walls (like Metroid's Wave Beam). Blossom's Fireball can melt ice blocks, but there aren't that many of them. Bubble's Ice Barrage can destroy the red-and-blue energy fields (but not the yellow ones).
There are some problems with the game however. 1. It's short, completed in 2.6 hours with 99.4% completion (I have no idea where that last uncleared room is). 2. It's relatively easy. I never died once, even on Normal, though I did give the New Game+ Mojo's Key Quest a try, which is not only harder, but gives you a new map, and I did wind up dying rather quickly. 3. There's a lot of screen tearing here. This isn't something I normally notice, but it's there. I'm sure more graphic-oriented people will be bothered by it as well. Unless there's an option I need to look at that fixes it.
The initial asking price of $7.99 (though it's on sale ATM for $6.39, and I paid $5.37 because of the $1.02 in my Steam Wallet), and I think that's a fair price. It's a good game overall, and I would recommend it for any Powerpuff Girl fan (even the kids, who wouldn't have any problems with this).