MacPro 2010 adding SSD drive
I have a 2008 Mac Pro and I swapped the main HDD with an SSD and re-installed OS X and I saw a HUGE performance increase. You'll always see a performance increase when going from HDD to SSD simply because it's a different class of hardware. SSDs are, as their name implies, solid-state. There are no moving parts and there's nothing that physically degrades over time. HDDs start to wear immediately; while it's a slow process it does happen for as long as the drive is in use (that's why most operating systems tend to turn HDDs off when not in use - it prolongs their life).

That's my experience and my recommendation (my recommendation is definitely do it).