After 25 years or so of slapping upgrades in computers of various sorts, I'd have to say an SSD made the most immediate, noticeable difference of any upgrade I've done. Better CPU? Yeah, the new one's a bit snappier...I think; or maybe I want to think that because I spent money. More RAM? Seems like it's not swapping as much, sure. Replace spinning platters with SSD? Did someone just secretly swap out my old computer for a new one? Everything seems faster (okay, not ripping DVDs in Handbrake).
Forget boot times, who reboots enough to even notice? App loading, compiles, anything involving disk access is nearly instant. I'll sacrifice capacity for what an SSD buys me.
Now I'll admit that I wasn't as impressed as I thought I should have been. Two years ago when I bought my first one, bloggers were wetting themselves a bit much over the extra snappiness of an SSD. But SSDs are still a damned impressive upgrade. I really noticed the difference when I went back and forth between my SSD-equipped MacBook Pro and my iMac with a better CPU but 7200 RPM hard drive. When the iMac hits disk, it's annoyingly noticeable.
In summary, SSDs have been worth the money to me for over two years now. The only spinny hard drive I'll be buying from now on will either be a secondary drive, or will go in the NAS.