Firstly, if you want to try some blades, I believe there is a Blade PIF (Pass It Forward) in the Lending Pool section - there are loads of different blades in there, so pop your name down and when it arrives, take one or two of each that you fancy, and send the rest onto the next in the list.
Anyway, back to your original post, I rate blades on:
Smoothness of shave - how much/little it feels like its scratching
Sharpness of blade - how much it tugs rather than slicing through
Quality of shave - how stubble-free your skin is afterwards and irritation levels afterwards
Longevity of blade - how many shaves the blade can manage before the above deteriorate
However, I did find that for the first couple of months of DE shaving, there wasn't that much difference, save for some awful blades that I know I'll never use again - namely Treet and Laser.
Now I've been a DE shaver for nearly a year, I'm finding that the differences between the top two criteria seem more apparent between blades.
Of all the blades, it seems that the Russian blades are most at risk of disappearing (similar to the way they stopped production of the Super Iridiums), so if you find you like any of the Gillettes, it would probably be worth getting hold of a few hundred.
I've settled on Astra SPs and Supermax Blue Diamonds as my main stock blades. Others come and go for a change, but I always seem to come back to them.