For desktops SSDs have never been more attractive. OS disk SSD and storage conventional HDDs. Not-new notebooks are a bit of a meh...
I have a reliable notebook... it has a 320GB drive that is nearly full... I was recently given a new 128GB SSD drive and tried to work with all the other stuff on an external via USB... easy I thought to make do on 128GB. I slapped the 320GB old drive into an enclosure.. and it was horrendous.
What I mean by this is the OS and applications were fantastically fast... the Windows 7 XP mode and other VMs were instantly accessable, but it became a shlepp working off an external USB... waiting for large files to open in photoshop etc off the USB drive... was tedious. I eventually started copying a lot of the frequently used source files from the USB to the SSD and then back again... which became self defeating as the SSD was constantly being written to and re-written to.
My options were a bigger SSD, new laptop with USB3 or external FireWire drive... all expensive options just to use the small 128GB SSD conventionally.
I removed the SSD and gave it to a family member... re-installed the 320GB and now sanity rains.
I have ordered an optical bay 2ndd HDD caddy thing for the laptop and when it arrives I will try the SSD route again, at least the magnetic drive will use the SATA interface and not USB... so it should be very usable.
I found that with notebooks 128GB just does not cut it... need 256GB or ideally a 320GB that will not need a kidney sale to fund.
EDIT: My brother is running a hackintosh... he is doing very nicely with his 128GB SSD as his OS drive, and has about 4TB of magnetic drive as storage... he can never go back to non-SSD OS. Family and friends are being evangelically converted to desktop SSD drives... some as small as 60GBs working perfectly with Windows 7 as OS drives and all the photos, email, music and movies sitting on the 1TB storage drives.
I guess it is all down to what you need accessible on your SSD most of the time. For a lot 90GB / 128GB cuts the mustard.
SSDs are phenomenal upgrades... life changing most of the time, provided you can work within the cost / size constraints.