it depends on how familiar you are with your calculator and your physics. For instance, if I know precisely what equations/concepts to use in a problem, I can get an answer out very fast on TI-89, since you can just visualize the equation in your head, (specially in relativity, all that square roots can be messy), then use the solve equation function in the calculator. However, If you do not know the physics, you are pretty much screwed up whether you have TI-89 or not.
The advantages I can think of about the TI-89 are:
1. It's got all the constants stored (c, h, Na, mass of electron, of proton, of neutron, ideal gas constant...) , and units conversions (ex: J to eV, miles to meters, AU to meters (this is the most frequently used function, at least for me)).
2. the algebraic system; it calculates integrals with symbols. It is a nice reference if you forget how to do an integral. Also, it factors polynomials and integers! Besides that, it has series and products so you can get some nice estimate for problems involving series or products.
3. Games. When I was in high school... boy, I got to have my 89 around... But now that I'm in college, I've erased all my games in my calculator.
Well, basically, comparing it to a TI-84, a TI-89 is not so much of an improvement. (the 3-d graphing is junk). I would recommend you to keep using and loving the TI-84 (I think it has more memory than the TI-89... )