If you are a heavy user/multitasker, it's totally worth it. I hate using computers without them now, once you get used to your PC working without an IO bottleneck It's incredibly annoying to use a spinnie again with all the little delays and choking up whenever you try and multitask.
It does vary person to person and system to system though, My mums Macbook was absolutely transformed by the upgrade to a Vertex2, Probably because it was a little RAM starved (1.5GB). She does a lot with iPhoto and things like browsing the iphoto albums in an open file dialogue box with all the thumbnails used to be painfully slow. I also upgraded my dads Vista laptop with a 160GB Intel - It now boots in 30seconds instead of over 5 minutes but apart from that he hasn't noticed much difference, which I put down to him being a fairly light user, and Vista having pretty decent pre fetching (he has 4GB ram).
I get by very well with 160GB space (150GiB after formatting) My bulk media and docs are all kept on my NAS anyway.