Performance is amazing. Speed increases in boot and application start times are immediately noticeable. My boot time went from about 1.5 minutes to 37 seconds. I also saw application start times on par with this. I would say boot and load times are down 30%-50% across the board.
This is many times an unexpected but appreciated benefit. I saw my typical battery life increase by about 20%. It makes sense - no moving parts = less energy to run.
My laptop was noticeably quieter. Not only is the drive itself silent, but my CPU fan doesn't have to work as hard. See below.
SSDs don't put off as much heat as a standard hard drive. Again, no moving parts means no friction, which means less heat generated.
SSDs are still expensive. I went with a 32GB drive to get it into my price range. Cost per gigabyte is more than double that of a traditional hard drive.
There are SSDs available at 256GB and larger (depending on the form factor), however they get exponentially more expensive for the larger drives. This will change as manufacturing ramps up and they start to take the place of platter type drives.
In my opinion, well worth it. I just ordered a new laptop and it will be getting the SSD upgrade immediately, but this will not be the case for everyone. If your laptop is your main computer, housing your iTunes library, thousands of photos and movies files, or gigabytes of documents, and you don't have an unlimited computer budget - an SSD is probably not for you at the moment. However, if you don't have to keep a lot of media on your laptop, it is probably the best speed upgrade you can make.