My jaguar windshield cracked a foot long from a wiper arm (it's metal!) during replacing the wiper blade. Why did I decided to replace it myself? For saving $, for learning, or for nothing to do during SIP. 
I got the parts ($225) from glass.net, tool set ($19) and primer ($20) and urethane tube ($35) from ebay. And of courses, hours of watching Youtube trying to find a method that fits me: metal wire, poke through under the glass, pull to cut the urethane seal. The urethane caulking gun was over a hundred dollars I was later allowed to return to HD.
Two weekends probably 20 hours as compared to 1 hour for a pro. The hardest is indeed the cutting the urethane seal. The cutting knife from the toolset was useless, dull and a skill I do not have, e.g., good angel to cut. The long poking sticker worked ok in pushing the metal wire thr the glass. The metal wire is very strong probably 10 times than a 40 lbs fishing line and yet it broke a dozen times.
I managed to replace the third brake light (it was dim, but works in the first place!), and drooping ceiling cloth and the vent (both failed). The protective moulding around the windshield and wiper spray. All the reasons why I wanted to replace the windshield myself.
I did solicit wife in positioning the new windshield.
If I were to do it again, I would find the right sharp cutting blade to do most of the cutting and use the metal wire to cut where the knife won’t reach. I doubt I would ever want to do it again.
