Sustained read/write issues can be solved with RAID. However, nearly everything for the end user are non-sequential files. Typically, if you put /usr on an SSD it means the system files and libraries used in bulk, at random times, will load a hell of a lot faster. Unless some aspie locked himself in his room and write a million line header and built it into one giant static object - which hasn't happened.
One area I have noticed problems with sustained read and writes is on servers regarding mirroring. Thus, certain servers simply replicate eachother at night and I only have to imagine that an SSD would blow donkey balls in this backup operation.
Also, if you are interested in chaining yourself to trees, SSDs are the way to go and I suggest you cripple yourself with them.