The CAS system is not allowed on many standardized tests, so that may be something you want to keep in mind. Also, certain teachers will not allow calculators with CAS, especially in university. I personally have found that a TI-84+, the TI-84+ CSE, and the TI-89 are the most widely accepted for use in high school and university. The 84 series allows for full TI-BASIC functionality, and is great not only for mathematical programming but also great for making games. The TI-89 I do not have experience with, but it uses 68k Basic I believe. Keep in mind also that there is a larger community which programs the 83+/84 series than programs for the Nspire, so if you are programming on the Nspire you may not have as much help as you would on the 83+/84 series.
Overall, the Nspire is a powerful calculator in some regards but a weak calculator in others. It all depends on how you want to use your calculator.