Upgrade iMac late 2009 with SSD
Here are some points from my Upgrade of my old iMac (late 2009, Model 10,1, #A1312) 


You can expect a significant Performance increase when starting Apps or saving Data, but overall-performance is depends on you RAM size too. If you are  working with Photoshop f.e. you should def. have more than 8GB (on 10.10) - the more the better.
You will encounter Problems with the Heat-Sensor. Apple uses a non-standard 3-Pin on the Hitachi HD to access the Heat-Sensor build into the Drive. Other HD or SSD will not have that, so you cannot connect the Heat-Sensor to the Mainboard. The result: the OS will turn up the Fan speed to a maximum, which is annoyingly loud. You can counter that effect with an additional Software to control the Fan speed, f.e. 'HDD-Fan Control' or 'iStat-Menu‘ and others.
In order to fasten the SSD, usually in a 2,5" Format, into the internal 3,5-Slot of the iMac you need a Frame for the SSD. Annoyingly some of the Frames have Screw-Slots in which the Apple Srews do not fit. You should read the excellent visual Tutorial from iFixit for replacing the HDD (Link is for my Model, check your Model before attempting to follow it).    


Conclusion: even though it's a PITA, i never regretted doing it.