I had a TI-89 I got in 9th grade and had through the end of college, but I found I had the opposite experience with it. A goodly number of my engineering classmates used their TI-89s as a crutch and forgot most of the details of solving e.g. differential equations and computing integrals. In my case I found it faster to do most of the problem solving by hand, and I just used the calculator to punch in the numbers to get the final answer. I found after a while that my TI-89 just sat in my backpack doing nothing most of the time and all I needed was a scientific calculator for most things. About the only time I found it really useful was for solving for stability characteristics and tuning tedious PID loops for control systems problems. Solving characteristic polynomials for poles and zeros was just a pain in the neck.
It had a nice side effect of saving my bacon several times on final exams. On my engineering electromagnetics final I forgot to change the batteries in my TI-89 the night before and the calculator didn't work. I ended up having to re-derive a small bit of transmission line theory from scratch in order to solve the problems. I somehow managed to be one of the first people done anyway. I did have fun making arcade game clones and custom boot screens in assembly on it though.
It had a nice side effect of saving my bacon several times on final exams. On my engineering electromagnetics final I forgot to change the batteries in my TI-89 the night before and the calculator didn't work. I ended up having to re-derive a small bit of transmission line theory from scratch in order to solve the problems. I somehow managed to be one of the first people done anyway. I did have fun making arcade game clones and custom boot screens in assembly on it though.