Yes it's worth it and very noticeably, even if SATA II slows the read and write speed down to about half of what it could be with SATA III, anyway it's much faster than what you get from the HDD that is now in it.
Also flash storage has a very much faster data access speed compared to spinning drives. So read and write speed is not the only advantage of SSDs.
I had a 2008 17" MBP, 4GB RAM, HDD, El Capitan.
Booting up the system took 1-2 minutes, starting stock applications like Safari and Mail had been slow too and I got "beach balls" when I did too much at the same time.
I replaced just the HDD with an SSD and it is running now almost like a new Computer, with still just 4GB of RAM.
(Comparable experiences I had with a Late 2009 17" MBP and a Mid-2010 13" MacBook.)
You have got a much newer faster CPU/GPU, double amount of RAM so the HDD seems to be the bottleneck, at least for most simple stuff (I don't know your usage).
SATA-SSDs up to 500GB are not very expensive anymore, you can even get 1TB SSDs for less than 2 x 500GB sometimes.