2D side Scroller game.

I'm attacking my first really big AS3 project, and I'm really going all out here.

But before I touch a lick of code, I want to know what the community thinks about a few things.

The first hurdle I'm looking at when creating a side scroller flash game (very lemmings like), is the physics. This game will have a lot advanced collision detection. But somewhere in the back of my mind told me to look at a physics engine, as they generally have great collision detection.

I could do this, however; I'm curious. If I'm doing a game where the characters fall off of ledges and have to realistically walk up stairs and all that.. is it best to use a physics engine? Or is that too much?

I can see it being helpful, and hurtful at the same time. It'd solve the whole gravity issue, but then I'd have to solve a lot of issues where I want the character to stay standing up and all that.

Also, if you recommend going with a physics engine - what'd be a good physics engine. I'm seeing APE, and fisix - but are there any other good engines?

What's your experience with this?

Thanks!

IMHO a real physics engine with collisions and everything brings way too much overhead to something as simple as Lemmings. Other than falling and colliding with a single object, and bouncing in a relatively predictable way, there's not a lot of complex physics going on... true, you have the cascade effect of one lemming pushing another, etc. ... but these are cumulative motions and don't actually work the same way as say, a pinball game.
The nature of the physics is more like Arkanoid, which is to say fake, almost nonexistent. I would work on code that replicated the basic types of motion you're trying to achieve, with a little bit of randomness involved, and write it so it looks good, rather than try to cajole a (relatively slow) physics engine like APE into handling your characters, which really isn't what it's designed to do...