Hello,
If you're on a Mac Pro, thinking about SSDs and 15K hard drives, be prepared to spend some money. If you need the speed, don't buy the cheapest member of a high quality category.
IMO, if you need real speed for your OS, hard drives, regardless of their speed, are obsolete. Many many people on these boards (including myself) have been using SSDs, and everything is fine.
Also, as someone else posted, boot times are meaningless: either leave your mac running awake 24/7 (like I do), or put it to sleep when you don't need it.
The same goes for app loading times. RAM is so cheap these days, and Mac OS X so efficient with it, that you don't need to close your apps as soon as you're done every time. Boot time and app loading time are irrelevant "benchmarks" or values these days.
If you're thinking about speed, regardless of the applications you use, the first thing to check is your processor speed. Then you go on to the RAM, and make sure you have enough. The rest of the upgrades offer only minimal gains compared to those first 2.
A SSD (or 15k drive) will NOT make your apps go faster in almost every case. Before buying a SSD for performance reasons, make sure that it will help your apps.
Loa