Is it worth to buy a SSD for my desktop machine if only support SATA II?

I wonder if I'd really notice the difference.

Hell yes. There is a big difference. Both in price, size and performance.

SATA-1,2 or 3 matters a lot less. Having an older SATA interface on your motherboard will slow down benchmarks and set a cap on sequential reads (e.g. reading a large movie).

However the big advantage of SSDs is random I/O. These kinds of access do not come close to the performance ceiling of SATA-2, let alone SATA-3.

You will most definitely notice it! I'm a developer as well, and a couple of years ago when I upgraded, even with the high price and small space of the SSD, I considered it the single best upgrade I had experienced in years.

If you upgrade machines in the future, you can bring your SSD with you.