Amongst those I know, SSDs are being used mostly as boot drives as this does speed up startup time considerably. They are also used for large sample libraries as loading times for them is reduced quite a lot too.
Whether they would speed up your saving times once you’ve done your hardware upgrade depends on what the bottleneck is. It might not be the hard drive. If the critical factor is the processing that you’re doing during the save, then a SSD won’t help, and often HDs are almost the speed of SSDs during saving / loading of moderate files due to their built-in RAM cache, so I’m afraid it won’t guarantee a performance increase unless the HD really is the limiting factor, e.g. with very large files and a straightforward save operation.
Although SSDs are coming down in price a bit now, another alternative is the ‘hybrid’ drive, which uses a larger RAM cache such as 4GB (sometimes actually a small SSD itself) and a spinning conventional hard drive behind it, which in a lot of circumstances approximates the speed of an SSD (or so the Kool Aid says… not tried it here).
The other advantage of actually going with a SSD is their totally silent operation, of course. Can’t wait to go all SSD myself once the price is right!