First things first, this product is just the glass, not the frame of the glass which means at some point you will have to separate the smashed glass from the frame which is quite messy and not for the faint hearted.
This involves warming up the glue with a hairdryer and detaching (picking) the glass out!
Some youtube videos show a more complete product which is very easy to do. Search "iPhone 3G / 3GS Glass Digitizer Replacement Repair HD Tutorial DIY Complete" on Youtube and you'll see whats involved. I couldn't find a tutorial using the tools supplied but they work and after watching the clip you get the idea.
My impressions of the glass is that it's not the official well made Apple glass, the sensors at the top are now clearly visible and it might just be me but it seems ever so slightly warped which means the squareness of the app icons look slightly different from top to bottom. I do mean very slightly, some people would not notice. the glass is also rough to start with, I sprayed furniture polish onto kitchen towel and then wiped it which has got rid of the friction. The home button is slightly sunken now but not too much of a problem.
If you are planning on keeping your iphone for a few more years and it's out of warranty then I hate to say it's probably worth spending the £80 for Apple to fit a high standard screen which includes a new LCD screen, they don't just replace the glass, hence the crazy price!
If like me you are just waiting for the iphone 5 or another phone to be released then it's definitely worth doing even if it's just to get one over the Apple "genius" snob who treated me with utter contempt at my pleads to haggle at the price.