SSDs are a waste of money at this point. For smaller systems where every extra bit of speed and performance is crucial (netbooks, nettops, etc.) then a small SSD is better than an HDD. Those systems tend to store very little data.
For bigger systems like desktops and full feature laptops, SSDs are a waste of money. It offers very little performance gains. All the benchmarks I've seen simply state "I can open X number of apps in like 1 second!" or something along the lines. There are no real world performance gains. If I open Word in 0.5 seconds as opposed to 10 seconds even, would I pay more than 5x the cost per GB? No.
I can get a nice 1TB HDD for about $100. I don't have to worry about disk space constraints. For the same $100, I can't even get an 80GB SSD. I'm no power user but I'm currently using about 100GB out of my 320GB HD. I don't have a photo library either.
Conclusion: If I were smart, I'd put that $100 into a better processor or graphics card. Or even better, into a bank account.
What do you consider as "real world performance gains"? It's not just opening apps, everything is faster using an SSD. If you use your computer all day for productivity, everything adds up at the end of the day. Luckily, I was "smart" and invested in an SSD which allows me to use "my" computer in a more efficient way. Your needs might be completely different, hence the justification of SSDs as "waste of money", and I'll respect that...