Getting a Ti-89 is kinda like giving a child all the answers to every problem in the book. If they’re good students, they’ll still make sure they understand the concepts, but if they get even a bit lazy, they can let the 89 do everything for them and end up not learning a damn thing.
I’ve been using the 83-plus since Trig, and ever since then I’ve found the calculator a useful tool to have with me. I’ve not been able to use it on any of my Calculus tests, but for homework, it’s a lot easier than having to take all the fractions, put them under the same denominator, and then adding them up. Yes, I can do that on my own, but it’s a lot easier to do with the calculator. In Chem, it was definitely nice to have, yes I could have used a scientific I suppose, but it’s a lot easier to manage with a multi-line display. The uses in physics are along the same lines. While it’s not ‘necessary’ for the most part, it’s a lot nicer to have, it makes things easier, at least it has for this second-year EE major. (And no, I have not grown dependent on it, in fact I’m pretty proud of my mental math ability, and can function very well without my calculator.)
I’ve used every graphing calculator TI has ever produced (and own an 85, an 86 and an 89) and I can say there is no reason why an 83 wouldn’t be more than enough for high school/college math. In fact, at my Uni the 83 is the standard requirement for all of the lower-tier trig and calc classes. Throughout four semesters of math classes I was permitted to use a calc but only needed to in the lowest level class. As an astrophyiscs major I hardly ever used a calc except to do some number crunching on problem sets and in intro pysics classes. The higher up you go the a calc becomes less useful whereas a computer program like Maple (my favorite) or Mathematica become the standard.
When I was in high school, the math department had a couple of class sets of 83’s that we could use in class only…it was reccomended that we buy our own for homework and for chemistry though. I still have mine, but use it primarily as a regular calculator. I do like having the large viewing screen though…it helps with my bills 
And I hated the games…every time I loaded one on, it wiped out my stored memory, and I always lost whatever formulas I had saved. But I think I’m just special…
I got a TI-83 for Algebra I my freshman year of high school, and I still use it to this day. It’s a little worn (the cover is wedged on with some folded paper, the rubber pads are gone, and it has several scuff marks on it) but I can’t imagine not having it. I’m going to be using it for my Stats class this semester of college (I’m a sophomore) and I’m glad I have it.
As for programming, I learned most of my high school math by figuring out how to program the calc. That’s what most people don’t realize: you have to know how to do it before you can write the program to do it.
Think of it as a long-term investment, and don’t bother with the 89. When it’s legal on standardized tests, pigs will fly.
College math teacher checking in here. I personally have never owned/used a graphing calculator, partly because I’ve never had $100-$150 that I wanted to spend on something that I’d just have to worry about getting lost, stolen, or broken, when I had a computer sitting at home that could more easily do anything I wanted to do with it except carry it around with me.
And in all the classes I have taught (i.e. freshman/sophomore-level college math classes, including all levels of algebra and calculus), I have never required any of my students to have a graphing calculator, partly because I felt bad requiring my students to spend that much on one, and partly because I didn’t want to spend class time going over how to use it. But I do encourage those students who already have one to use it to check their work, play around with it, etc. And many of the textbooks I use have bits in them on using the graphing calculator, that can be read or ignored at the student’s discretion.