I'll clarify for you all: This is the best modification you can do to your computer - bang for buck.
Warranty: I have been performing SSD upgrades for a few years and have constantly been in touch with Apple service centres and Apple executive relations. The status of warranty is void once the machine is altered, due to not knowing if a part was made faulty or turned faulty due to modification/opening of the machine.
I have argued with Apple that they have 'void' warranty stickers in place inside the iMac on the CPU Heatsink which to me implies that you can open the iMac but if you remove the heatsink it's all void. It took 2 weeks to receive an answer from Tim Cooks office (Executive Relations US) to clarify that the sticker is present but any opening of the machine by anyone other than an authorised service provider will void the warranty.
They further explained that if you have an authorised service provider install a third party device such as SSD then this will retain your warranty on the condition that the install is registered on the repair database available to the authorised repair network/apple stores.
In the case you meet the above and it is recorded you could take a defective machine to your local Apple genius whom upon reading the notes of the authorised part installation could perform warranty work on the machine, with the exception of any after market parts that were not present at manufacture.
This is Apples official stance on the matter. If you want to query any of what i've said by all means contact Apple and go through 17 advisors who do not know for sure, some know definitively but are wrong, eventually you'll get higher and higher until executive relations respond with the official stance.
SSD install is easy for most comfortable with replacing a hard drive, if you want your warranty in-tact use a service centre accredited by Apple and have it recorded (You pay to have it installed but in most cases can provide the part) or you install it yourself but remove the drive before any warranty claim, not entirely moral but a choice.
I hope that helps some people on the whole warranty is void/not void issue as it's not so cut and dry so to speak.
