About HDD space: My HDD is 160 Gb, and while that may not be much for some, it's plenty for me. Beside, if I decide to keep an ISO after testing it, and I happen to need the space, I'll burn it for storage purposes (see JJXB's post just above).
I also think that it's faster to read from the HDD, and the HDD doesn't make as much noise as my CD-ROM drive. It's hard to concentrate on the game when you constantly hear the grinding of cogs and gears.
Last: With ISOs on my HDD, I don't have to keep a stack of cds within reach. No need to haul myself over to the rack to get that cd. I just point and click, and there it is
