My online experience has been a mixed bag. Lag destroys most of the enjoyment of the online races, as people are never quite where they look like they are on your screen. It's not a constant thing, but it does seem to happen when you're needing it to. Aim is impossible, so online strategy revolves around laying traps rather than throwing bombs. Doesn't change too much for most people, but it is frustrating to finish in "1st", then have it change to "3rd" when the result board pops up.
Offline is amazing though. Loads of fun, difficulty is nice, and each cart and character have a different feel to them. I use to be a light character racer before, but have found mid-light are much better for me in this iteration.
Use a USB-LAN adapter and your lag issue is solved. The CPU is either perfect or a total asshole (i.e, all hitting you with items unexpectedly) and it's called a kart, not a cart.