On Macbook Pro, how to tell if my hard drive access speed is a bottleneck to my current computer performance?
With Yosemite, we're noticing an somewhat, but not totally, subjective slowdown to all aspects of the OS, despite the minimum 4 GB RAM.

At our school we are actively replacing any faculty member's MBP's internal HDD with a SSD if they complain of 'slowness' and they can demonstrate it concretely. This can be in the form of a slow startup; slow-loading apps; frequent SPODs; anything which gets in the way of teaching. Stock MBPs use slower 5400 RPM drives, which is where some of the problems originate. Even updating to a 7200 RPM drive can help alleviate the perceived slowness.

Granted, some of the slowness can be attributed to never restarting the MBP, ever; having the ENTIRE Office '11 suite of apps open at startup; few free GB on the HDD; and similar user issues, but we've found the complaints go away when the HDD is replaced with something faster.

The only complaint we get after the swap is that the SSD we offer is usually 1/2 the size (or less) of the stock HDD, forcing users to move their iTunes libraries onto external disks or actually cleaning out the cruft from the HDDs. To alleviate this, we put the removed HDD in an inexpensive USB external enclosure as part of the deal.

Go for the SSD upgrade. You won't look back.