I've had both the Cameleon and the Donkey and TBH you can't go wrong with either. With DS1 we got the Cameleon and I loved it! I actually used to joke that I wouldn't have a second child until Bugaboo came out with a double LOL.

We got the Donkey while I was pregnant, so I've used it as a single quite a bit with a toddler. The side shopping basket is brilliant and overall the pram handles just as beautifully as the Cameleon did. The only thing I like better about the Cameleon was the tyres, they were foam filled so you never had to inflate them. The Donkey has airfilled ones which you have to top up every 6 weeks or so and that is annoying. We had DS2 two weeks ago and have used it as a double with the MaxiCosi capsule a couple of times, and it handles just as beautifully. It really is a great pram.

To answer your questions...
Does it push well?
Yep, can't fault it!

Is it worth the price tag?
I think so. Bugaboo stuff holds it value incredibly well. I sold my 2.5 year old Cameleon with an extra set of topping for $880. I'm expecting that I'll also be able to sell the Donkey for a good price when we're done with it.

What about the fold?
I don't have a problem with it. Again, it's pretty much the same as the Cameleon. I personally like that the seat detaches. At our old place I had to keep the pram in the car so I would put DS in the bassinet/seat and then unfold the frame and pop it on with him in it. My neighbours who had a single fold pram had to put their LO in their car sear while they got the pram out which was a real pain for them.

Is the mono really a compact fold as advertised?

Looking at it, you wouldn't think it is the same width as the Cameleon, but I actually had them both side by side and can attest that the wheel base is the same width.

Personally, I think you can't go wrong with either. But if you're going to use a double (even sometimes) then for the extra $$ the Donkey is worth it. I have 2.9 years between my boys, so I'm guessing that in a year of so's time DS1 won't be going in the pram, but for the time being I couldn't get by without a double. But, I do a lot of walking and like to go to walk to the grocery store each day etc. If I was someone who used the car more often, then I'd probably get by without a double.

HTH