The boot time is just a convenient "wow" factor, the real benefit comes from the thousands of little everyday tasks becoming CPU-bound instead of IO-bound.
I recently replaced the 5400rpm drive in my macbook with a larger capacity 7200rpm drive, and the difference in performance was quite noticeable. Of course no where near the jump from 7200 to SSD, but still a decent jump. Downside is that the new drive is loud enough to hear, whereas the original drive was dead silent
Yeah, I did the same before jumping to 100% SSD. Also, heat/power consumption.
If you need the storage of a spinner and can't afford a big enough SSD, running a small SSD for the system/apps with a 7200 RPM as a storage drive in the place of the optical drive works pretty well. Look for the cheaper versions of the OptiBay (which is a bit of a ripoff).
A 7200 RPM drive is absolutely not comparable.