I've had an SSD for the better part of a year and really notice the difference when I have to work on a system with an old-school mechanical drive.
Aside from the obvious boot time improvement, I think what really makes a difference is the swap file, which is already completely random-access in nature. Microsoft introduced ReadyBoost in Vista for this very reason, but the performance of a USB thumb drive doesn't even begin to compare to a SATA3 SSD.
So you'll notice a huge difference if you're doing anything memory-intensive; doesn't matter how much physical RAM you've got, there's always going to be some paging going on, and an SSD makes that almost invisible.
That being said, if you do plan to get an SSD, make backups. I thought SSDs were supposed to be more reliable than mechanical drives - due to having no mechanical wear and tear, obviously - but less than a month ago, my drive (Vertex 2) suddenly died without any warning. I mean completely shitcanned, just blue-screened one day and the computer refused to recognize it after the reboot. Could not resurrect, the usual Linux Live CD standby and even the manufacturer's firmware tools wouldn't help.
I subsequently found out that the older OCZ drives (prior to series 4) have a reputation for this, as do all SandForce-based drives (which is... almost all of them on the market), and therefore chose to replace mine with a slightly older model that has a proven track record (Plextor M3). Still, I feel a little like I'm walking on eggshells.
Even Intel, whose original X25 drives were rock solid, has recently switched to SandForce technology, and (surprise surprise) users are reporting a lot of problems with their newer drives (330, 520, etc.), specifically that "8 MB bug" which they've supposedly fixed, but reports are still coming in that the fix isn't 100% effective.
So, expect a noticeable performance improvement, but just be prepared for problems. Fortunately, since throughput is so high, you should be able to do a full restore in a matter of minutes - as long as you have a recent/recoverable backup!
My party schedule:
2009-02-21 - DJ Attention @ I'm So Popular
2009-06-18 - DJ Annoying @ People Need To Know Where I'll Be
2012-11-32 - DJ Insufferable ɸ Or At Least the Stalkers I Complain About
2048-06-66 - Spastic & Whocares ¶ Although I'm Actually Flattered
9999-45-81 - Tweaker Gimp ☼ I Probably Won't Even Go To This But I Have To Make Sure I Fill Up All The Available Space Here