An SSD did improve battery life slightly and lowered temperatures by 1-2 C in my PowerBook. Speed increase was noticeable (especially when booting) but not mindblowing.
There are other reasons to use SSD's other than performance. Power draw and durability are two good ones. As for performance I can tell the system boots / shuts down and opens things faster. But, IMO, these are things which don't happen often and don't consume a whole lot of time to begin with. IMO, without a specific reason otherwise, one should not expend a great deal of effort / money to improve these things. With that said SATA SSDs are coming down in price and low capacity versions can be had for a reasonable price. The problem is the iBooks aren't SATA based.