The SSD is definitely worth if for a laptop. The performance increase is unbelievable. In a desktop if you are running a Velicoraptor then upgrading to an SSD it is pretty good. If you are running a Western Digital Green then it is very good. It is all relative. I finally went to SSD and I must say I wouldn't go back but you can survive with a fast HDD if your finances are limited.
Completely relative, it also depends on the SSD you get, for example Crucial M4 vs. OCZ IBIS. As far as SSD's go thats like comparing the WD Green to the Velociraptor. With SSD's you need to look at your sustained speeds, and your IOPS. Also going to depend on if your connecting to 3Gbs or 6Gbs SATA, and if you wanted to go really fancy the IBIS skips SATA all together goes with a special card in the PCI slot. IMO I would go with a reasonable OCZ if your funds are limited, I found an open box 100GB IBIS on Newegg it is by far the fastest thing I have ever seen.