Ravit, here's my look on it, I've tutored countless students with nspire (big bulky white one with green circle keys) and the new CX (coloured screen ones) and I've personally used TI-89 my entire life.
I love the TI-89, not biased because I've used it my entire life but because I've also learnt the nspire and CX myself from teaching. The TI-89 is easy to type and use, and if you're familiar with the setup you can use it with great efficiency.
The nspire is the one I hate the most, it's hard to use, keys are hard to type with, also alot of it's functions are set out in a way that lacks efficiency, I prefer the 89 over nspire anyday.
The CX, however, is fucking amazing, I actually wanna use one of these instead of the 89. It's easy to use, the keys feel great when typing (it gives me the feeling of actually typing on one of those 'crunchy' keyboards), the functions are set out nicely, good templates and it's COLOURED. You know, with the 89 and nspire, how many times have you used it at night at your study desk and the lighting isn't good enough to see what you're typing or when it's too bright and you can't see what you're typing. With the CX, the graphics are truly amazing. Also, it's rechargeable, meaning you don't have to keep buying 4 pack AAA batts.
All in all, the 89 is more than sufficient for VCE (and even in uni), and if you're already familiar with alot of the functions then keep it. If you do wanna get a new calc, then think about investing in a CX, the functions are a bit different from the 89 however it shouldn't take too long to learn if you're already good with the 89.