All I can offer is my personal experience.
Last week I took NewEgg up on their combo deal for a 160GB G2-M & 40GB G2-V Intel drive ($549 for both, free shipping). The 160GB went into my desktop the 40GB went into my laptop.
Wow...
All I can say...
I've upgraded, built, etc. so many computers over the years, it isn't funny... This is hands down, the single biggest upgrade I can think of since my very first hard drive in the 80's (30MB, before that it was a pair of 160KB 5.25" floppy drives).
For a few hundred bucks, my computer just got 50 times faster anytime it needs the hard drive. No CPU/GPU/Memory upgrade has ever done that.
Windows 7 boots in 20 seconds from power button to complete desktop, shuts down in 5 seconds flat. It installed Windows 7 in less than 5 minutes from 1st USB memory stick boot to finished desktop, including all reboots. Office 2007 SP2 installed so fast, I thought it failed. Didn't time it, but it felt like it was less than a minute.
Game launch time is MUCH faster than before. First game I fired up was Mass Effect 2, at least 3 times faster to get into it, no lag or loading time or stutter. Other programs like Adobe Photoshop CS4 and Office 2007, almost instant.
It isn't about benchmarks, they don't do it justice. It is like the difference between a single core and a dual core CPU, the whole system is just, snappier, more responsive. All the lag is gone from the system. The single slowest part of my computer is now gone, and it is noticeable.
The little lags here and there, the pauses anytime the system touches the hard drive are gone. I have other computers with hard drives still in them, it is almost painful. I expect that after awhile, I'll end up with SSDs in everything because once you've experienced it, you just can't go back.
These are expensive, to be sure. Anand is not kidding when he says this is the single best upgrade you can do to your system.
The laptop is a Dell Vostro 1720. It had a 320GB 7,200 RPM hard drive in it (now the second drive). Installed Windows 7 clean on the 40GB Intel drive. Hibernate is now USEFUL... It comes in and out of sleep fast. It isn't as fast as the 160GB drive, but it is close enough, and it is a bigger overall improvement because the hard drive in my desktop was so much faster than the laptop drive.
The real proof is this, if I had it to do over again, would I spend the $549 again? Yes, every day of the week and twice on Sunday... Wish I had done it last year when I first installed Windows 7, it is worth every penny if you can afford it (not everyone can, I understand that)... Hard drives cannot go away fast enough.