As someone who does try to train seriously for part of the year to get better - I have to admit that as cool as the GPS functions are, the mapping features are nearly useless for me.

A bike computer that measured speed, distance, and HR captures all of the critical values I need for serious training. (A Powermeter would trump this, but that's $$$$, and I don't need it yet to keep improving.)

There are two main benefits I've gained from the GPS mapping features:

One is elevation measurements, but even that is rendered pretty much useless as I can tell how hard I've worked on any course by seeing the amount time I've spent in zone 3-4-5 HRs. I think that my zone 3-5 HR data is the best measure of how hard I've worked out there short of using a PM. I can even tell if it's a sprint day by seeing how much time I've spent in the red-zone 5 (usually a lot.)

Second is getting a sense of my current fitness. If I repeat a typical nondrafting 50 mile course, I can see what my HRM/speed correlations are on certain areas (thanks to GPS) and tell how fit I am. I don't do this frequently, though, and I can easily live without it.

If I were short on cash, I'd definitely forego the GPS features and just get a regular bike computer + HRM. I'm actually finding that the time I spend in HR 3-5 is the single biggest indicator of my training, to the point that I could probably just ditch the computer and jsut go with a HRM. But it's nice to see how fast you're going.

The Garmin Forerunner 305 is a big watch that does HRM+GPS+computer downloads, without any additional hardware, for $150. They have newer versions, but I think this is the best bang for buck, and is what I use. It's awesome.