I just rented the game. And I had some of the same issues everyone else if having...but I have faired much better.
Here are some hints I have found.
1) When you are wanting to do an Uber Trick ready yourself for the trick by positioning where you want to start the movement before the jump. Remember to hold A before starting the movement. I usually press A for a 1/2 Second before starting the movement, and remember to let go of A after finishing the movement completely. Practice makes perfect, but I have found usually the error is my fault because I get to anxious while trying to do an Uber Trick.
2) Another tip for the game, is do Uber Tricks you know you can pull off when you are racing for tournaments or any real competition. Don't do the ones on the screen if you don't know them, and don't practice new Uber Tricks you haven't mastered on real events. Sure your character won't do cool flashy tricks, but you will save yourself headache from losing too often.
3) Slalom is easily the hardest event in this game. It requires precision racing, which can be hard with the nunchuk. To do the first race, the easiest way is to do large sweeps around each flag in the middle section, this gives you enough turn radius to come back around. Slowing down also helps, but isn't necessary. The course isn't hard, but you are fighting the responsiveness of the controls, which leads to hint number 4.
4) Change the control settings for steering. In the Control Options menu you can fine tune the controls to your liking by pressing 1. This is important, because if you do not like using the Nunchuk for movement you can bi-pass that completely and set it to use analog stick only. This makes the game much more responsive, and feels more like traditional SSX. It also doesn't take too much away from the Wii immersion of the game, because you still have to tilt the nunchuck to grabs.
And that is where I am having problems. The grabs are quite difficult for ME to do. I am sure others are getting it down well...but I am having difficulty. But grabs are not necessary to move on through the game...so I am still enjoying my experience.
Right now I am really enjoying this game. You get out of it what you put in. The game's control is deep, and EA really did several things right in this game. It isn't for everyone, but for those dedicated I would score this game in the 7-8 range depending how much you like the SSX experience.