It gives you analog and DV input/output, real time editing and ability to output to a TV (and viewing yer project on a TV screen is crucial if yer at all serious 'bout good looking video). It also offers accelerated rendering of the movie for final output, and real time output of After Effects to a TV monitor (fantasic feature if you have AE). Some people will use the RT more than others, but if yer not stoked 'bout any RT, at all then I don't think you've done much editing
Waiting to render sux. The 2500 can handle two layers of video and a text layer in RT. No waiting for titles/credits to render. No waiting for transitions to render either. I think w/the Pro Pack you get RT color correction too, but I can't remember. At it's price point you can't be the RT2500. Using my RT2500 is the only thing I miss about cutting on a PC (I bought a G4 2 months ago to be my new edting workstation).
What kind of editing do you want to do (just mess around, some home movies, or seriously get into editing)? Are you going to be using DV? Will you ever need analog inputs/outputs? What's yer budget?