Summary: Bought it for a 20", mid 2007 iMac which had 4GB of RAM and was prone to spinning beach balls. Replacing one of the 2GB sticks with a Crucial 4GB increased my Mac's memory by 50% and made it usable again. My only complaint is that I wish I had known the iMac's real memory limit earlier. Would have saved me a lot of aggravation and waiting...
Summary: When I when from Snow Leopard to Snow Lion on my aging iMac (just to read larger hard drives), I noticed the system seemed to lag a little bit. While Apple officially maintains that this model can only support 4 GB of RAM, I'd been told that with Snow Lion or a newer OS, it could read up to 6. So I gamble $85 and bought this 4 GB module and swapped out one of the existing 2 GB modules and the system does seem to run a bit faster! And the new OS sees the additional RAM.
Summary: I recently discovered that 2007 iMacs can effectively use 6GB of RAM even thought the specifications state that they max out at 4GB. I swapped this in for one of my 2GB sticks and now have 6GB of RAM working perfectly. The computer is running quicker and had no problems updating to OS X Mavericks (which I've heard may be a strain with only 4GB of RAM).
Summary: Bought this to increase my girlfriend's Macbook (early 2008). She was told by the Apple "genius" at the store that her model could support up to 6GB of RAM, so I bought her this single stick of 4GB, thinking that I could pair it with 1 of the existing sticks of 1GB that came with the laptop. However, while the Macbook booted up with 1 stick of 4GB and 1 stick of 1GB, it only recognized the 1GB.
Summary: Upgrading my MacBook to 6gb (this 4gb card and a 2gb card, which is the max is can handle without sending it into a tailspin) kept me from having to buy a whole new computer. Overall, totally worth the investment!