Agree.
I swapped my old Core 2 Duo E7200 (2.53) O/Ced to 3.8 with an OCZ Agility2 SSD for an i3 with a Kingston SSDnow 300 (*old* retired machine that was given to my dad). Altough the new machine boots way faster, and the new SSD is about twice as fast in benchmarks (even if low-end), I find it faster, but not *blew me out of my chair* faster.
Once you go from HDD to SSD, even the cheapest lowest performing SSD is gonna be much faster than anything with spinning platters.
True, that is simply "diminishing returns". Just going from a HDD to ANY SSD will make your computer incredibly faster, but then going to any faster SSD will not give the same benefits, because that one only will be faster on continuous access (like copying large files). Booting the OS or accessing small random files will not benefit much anymore. So going for a super expensive SSD will only be worth it if a.) you read/write lots of LARGE files (e.g. movie editing) or b.) need the long-term reliability of a SSD designed for multi-year writing of tons of data.
I bet you stuck to ATA for your hard drives, because you were used to floppys, and those ATA dries were soooo much faster, everything beyond was just diminishing returns.