Macbook Pro 5400RPM / 7200RPM and the effect on battery life
In general there is very little difference in power consumption between 5400 and 7200rpm drives. You'll need to check manufacturer specs for the drive you have in mind, as they vary, and a newer 7200 may even use less than an older 5400. But in the overall scheme of your MacBook pro, the differences are so small as to have a very negligible effect - possibly a few minutes in worst case.

The benefits in performance will definitely out-weigh the power issues.

That said, when I tried a 7200 in my previous MBP (Late 08 Unibody), I ended up putting the stock 5400 back in as I'd gotten used to almost silent/vibration free use of the machine, and the newer drive was noisier and vibrated a little bit more - now it's all relative, it was still better than my Dell work laptop, but I ended up putting it in a case and using it as an external backup drive.