Not really too much I can add to what has already been said. Macs are awesome. I love mine.
I have a Macbook and I HAD a iMac, but the screen failed on me and I got a refund. I may consider the iMac again when they sort out their screen issues. Until then I will stick with PC for gaming and my Macbook for emailing, graphics, movies, photos etc etc.
One thing you will notice with a Macbook / Macbook pro is they tend to last for years and years. Where friends of mine have replaced their Laptops at least twice, I am still running my Macbook and it feels the same as the day I got it.
As for upgrading, yes an iMac is not upgradeable other than the HDD and RAM. If you are into tinkering with your hardware and upgrading, don't buy a iMac, you won't get any satisfaction out of it.
As for the Macbooks, yeah you can upgrade to an SSD and more RAM and you will have one very fast machine.
