[Conor Ryan]

If you want to make money: HP 12C, thats what wall street uses.

If your going to be a professional engineer or planning on being a mathematics major, HP 50G is the way to go, its efficient and on the theoretical end is much more powerful, and will probably will save you a signifigant amount of time.

If you don't know what you want out of life yet, haven't gotten to Calculus yet, stick with a TI-83/84

If you have gotten to calculus and are at the point where you can explain in layman's terms what a partial derivative is, its about time for a TI-89. Professors at lower levels often do not allow these puppies on exams, in fact some math accreditation systems prohibit their use.

Its all about having the right tool for the job. Using a TI-89 or HP 50G in a spot where TI-83's will do the job faster, its not a smart choice to get the more expensive one (even if you think it will pay off over time as an investment)