I'm going to disagree with the negativity on this thread. While I don't use a mylar bivvy as an every night combo with a crappy bag, I've used it countless times when the weather turned out worse than expected. I've had great results. I take my best bag for the weather forecast, but in a 5 day fall backpacking trip there's often a night or two when I want something warmer than what I brought. Yes a mylar bivvy collects condensation, but the inside of my sleeping bag has always been dry. I would expect more problems if you're a restless sleeper or if you're freaking out about it and shaking it onto yourself, but the condensation is manageable. The condensation sticks to the inside top of the bivvy, it looks like more water than it is and it's not difficult to peel the bivvy off your bag without getting wet. Definitely don't pull it over your head and in the morning make sure to pull it off to dry. Let it and your bag air out during the day.
One reason I don't usually have condensation is that once I get warmed up in my bag I usually end up pushing it down to just around my feet, but that still makes a huge difference.
If you're really worried about a little water, mylar blankets work too, but probably provide about half as much heat retention and easily slide off in the night. If I go with blankets, I use one as the footprint under my tent as well as one loosely over the top with one end tucked under my feet.
I do agree that you should buy the best bag you can justify for the conditions you expect the most, but having some mylar (blanket or bivvy) in your pack can make as big difference on a cold night without adding much to your cost or pack weight.
