Just as an additional information/confirmation:
Installed multiple SSDs in multiple "old" MBPs and one MacPro during the last years:
MBP 2009 2,53 MHz 2-core:
Original apple 512 SSD (it is a Kingston from an 2011 MBP) : Works well (Write about 150 MB/s, Read 180 MB/s) due to older SATA connection of the 2009 MBP not too fast - but nevertheless more than double as fast (and very remarkable faster though) than the seagate 750 GB HDD before (about 60-70 MB/s write and 80-90 MB/s read on a NEW and nearly EMPTY HDDs, will saying under best conditions possible) ---> so: SSD still REALLY worth investing and fastening the 2009 MBP - even with 4 GB RAM (which should be upgraded to 8 GB - this costs nearly nothing)… this "old" MBP rests still usable and is sufficient for standard work without any problem. You really can avoid purchasing a new MBP if you do standard usage (Office, Web, regarding TV/Videos, and so on) if you just invest in a SSD and upgrade to 8GB RAM!
as stated already by others: the simple installation of a SSD and upgrading to 8 GB RAM gives your machine a new life!
the Samsung SSDs will also work without any problems but due to the old SATA-connection of th e2009 MBP also with reduced speed of SATA I (see above)
multiple MBPs 2011 and MBPs 2012:
Samsung 840 Pro 512 and Samsung EVO 1 TB and Samsung 850 EVO 2 TB: running well (because of SATA III with Write speed 490 MB/s, Read speed 510 MB/s) - firmware upgrade strongly recommended though to assure long-lasting performance. Works very well with a DVD-R on which the firmware-update from Samsung has been copied. (IMPORTANT NOTE for Users with other keyboards than the english keyboards: to confirm "YES" during the update process, you have NOT to press "Y" as told during the update-process, but to press the SAME BUTTON on your own keyboard which is localized where the "Y" button is localized on the english/US keyboard, i.e. "Z" on German, Swiss or Austrichian keyboards and so on…)
ALthough people wrote that you are not absolutely forced to enable TRIM, I recommend it for older OSX versions like Mountain Lion (which I still use). It seems, that since OSX 10.11.x TRIM is enabled by default. This should be controlled by searching in "about my Mac" : Search for "TRIM enabled: YES" .
Even Photoshop and other demanding work is fast with these MBPs - in you use Photoshop and other demanding apps, upgrade to 2x8 GB RAM is recommended (costs less than 90 USD/EUR).
MacPro 2009 (4,1):
All already above-mentioned Samsung SSDs work flawlessly also in this machine - in all HDD bays because of the limit by SATA II "only" with about 250 MB/s Write/Read speed - but in PCIe-SSD-Card solutions (I use the Sonnet Tempo) bootable and with full SATA III speed of about 500 MB/s write/read. Make sure that you chose the SSD as the booting device. About TRIM: same as above.
in general:
Installing an SSD after cloning your old HDD on it and upgrading RAM makes you - as others already stated - believe that you bought a new MBP! And the exchange itself is easy and fast to do.
All you need to do is cloning (for example with CarbonCopyCloner, which works self-explaining and flawlessly) - which takes some hours, depending on the amount of data to be cloned.
If you want to test and re-assure yourself about Write/Read speed use AJA system test or Blackmagic disk speed test.
While TRIM Enabler works very good, its disk implemented speed test has a bug concerning its additional feature for disk speed test and will give you nearly zero speed. The developer of Trim Enabler is informed and knows it. It is nothing but the speed test which is not optimal, the rest works flawlessly! And the developer deserves a donation for the excellent TRIM Enabler. I did so.