Yes, I have upgraded my MacBook Pro 5,1 (late '08 unibody) by increasing the RAM and changing out the hard drive. I did that about one and half years into ownership because I could. Have I ever wanted to upgrade the memory on my iPhone or iPad? Yes, but instead I select the device that I feel will max out my experience for the given life. Am I sad that I can't upgrade the new MacBook Pro with Retina display? Maybe just a tad because I cannot purchase a model with 16GB at the store unless I select the highest model. I can't just buy the one with 8GB and upgrade it myself but boo who, I'll just spec it out myself online and be at peace. Rafe pretty much sums its up beautifully.
"...I was very pleased to be able to take it apart and replace that hard drive myself. That being said, I’d rather have one of the new MacBook Pros with Retina Display than that old MacBook any day of the week." -Rafe Colburn