I'm trying to make a platformer with Flash/AS3 and have been looking for something to help with some of the collision etc. I've look at a few engines and Box2D/WCK seems the most sophisticated. Do I need that level of sophistication or should I just stick to something simple like Fixel or Flashpunk? OR should I just do everything myself?
Box2D is a physics engine; Flixel and Flashpunk are game frameworks. You can use both Flixel and Flashpunk with Box2D.
I highly recommend using a framework like Flixel or Flashpunk. It will simplify and accelerate your coding experience.
For a platformer, Box2D is probably not necessary unless you want interesting physics to be a part of your gameplay. If you do, Box2D will probably be useful, but I recommend using it on top of a framework.
Doing everything on your own will make you learn new things, but it's always better to not to reinvent the wheel. I've only used
Box2D in my projects and let me tell you, it's so easy to use that you'll never want to do collision coding on your own.
You may fall into performance problem, but it's easy to solve them (Mainly you'll get lower performance only when you'll add lot's of bodies or when you chose wrong scaling factor. You should read official faq for that).
You can also try with the Citrus Engine, http://citrusengine.com which it's specifically made for platform games and implements box2d for the physics. It's also open source
It's hard to know what is best for you, but I'd recommend you target the GPU for better result and take a look at starling.
It's painstakingly easy to hit the wall in terms of performance with the software renderer in flash player. Particularly to get a decent frame rate.
Some people have successfully implemented box2d in Starling, that's what I would look at using personally.