For the BMW, it turns out the warranty guarantees their work against leaks, defects and any installation errors for as long as I own the car. It's unclear to me whether this covers damage to the car, but I kinda doubt it. I think they are just saying they will replace the glass and repair installation errors.
Personally, I really don't think this is going to be a problem. I watched the guy install it in my driveway and it looked first rate. Plus, when weighed against paying $1700 for an oem and dealer install, I'm willing to take the chance (assuming that figure is right - seems way too much to be true though).
Oh, I see it comes with a 1 year free chip repair too - that's nice.
Sorry if it sounds like I'm advocating non oem installations -- but I'm really not. I replaced the BMW windshield for $250 with non oem because that car was just about out of warranty anyway. The facts concerning what the warrantees really cover are still vague to me. We are guessing at what the dealer cost is and what their warrantee would be. If/When I need a new FX windshield, I'll be making many phone calls.