We also have a Nuna Mixx (think Next is the newer version), and it's been a really good system for us.
It handles well over pavement, grass, and dirt paths and is nimble enough to move around shops easily (and one-handed), although it's not what I'd choose if you're going to be walking up muddy farm tracks every day. I find the front wheels do clog occasionally with wet leaves in autumn – I just use a stick to push the blockage out, and for us, it really has been only occasional.
It's fine on our local trains and buses, but I wouldn't want it (or any big travel system) on a crowded bus if I could avoid it. I generally use a carrier when we go into a larger city, and will use a small pushchair once DD2 is older. We rarely take it in the car (we rarely use the car), although it does fit in our small car, and I think the new version folds more compactly anyhow. It's also not the system I'd choose if you need to store it folded at home – it's not freestanding when folded (well, our version isn't) and the carrycot is really pretty big. The flip side of that is that it's a good system for long babies, and ours were unexpectedly 98th centile for length, so I shouldn't complain too much about the size of the carrycot!
The big basket is great if you run a lot of errands on foot, and ours has withstood a lot of abuse over the last three years and two children. The hoods on car seat, carrycot, and seat are also very good, and the magnetic sleep shade things are really convenient. My nosy children have both appreciated the mesh windows in the sides of the hood so they can still watch the world go by with the hood down.